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added a guide on writing good cucumber tests for osrm

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# Testsuite
OSRM comes with a testsuite containing both unit-tests using the BOOST library and cucucmber.js for scenario driven testing.
## Unit Tests
TODO write a guide on unit tests.
## Cucumber
For a general introduction on cucumber in our testsuite, have a look at [the wiki](https://github.com/Project-OSRM/osrm-backend/wiki/Cucumber-Test-Suite).
This documentation aims to supply a guideline on how to write cucumber tests that test new features introduced into osrm.
### Test the feature
It is often tempting to reduce the test to a path and accompanying instructions. Instructions can and will change over the course of improving guidance.
Instructions should only be used when writing a feature located in `features/guidance`. All other features should avoid using instructions at all.
### Write Tests to Scale
OSRM is a navigation engine. Tests should always consider this background.
An important implication is the grid size. If tests use a very small grid size, you run into the chance of instructions being omitted.
For example:
```
Background:
Given the profile "car"
Given a grid size of 10 meters
Scenario: Testbot - Straight Road
Given the node map
| a | b | c | d |
And the ways
| nodes | highway |
| ab | primary |
| bc | primary |
| cd | primary |
When I route I should get
| from | to | route |
| a | d | ab,bc,cd,cd |
```
In a navigation engine, the instructions
- depart east on ab
- in 10 meters the road name changes to bc
- in 10 meters the road name changes to cd
- you arrived at cd
would be impossible to announce and not helpful at all.
Since no actual choices exist, the route you get could result in `ab,cd` and simply say `depart` and `arrive`.
To prevent such surprises, always consider the availability of other roads and use grid sizes/road lengths that correspond to actually reasonable scenarios in a road network.
### Use names
If you specify many nodes in close succession to present a specific road geometry, consider using `name` to indicate to OSRM that the segment is a single road.
```
Background:
Given the profile "car"
Given a grid size of 10 meters
Scenario: Testbot - Straight Road
Given the node map
| a | b | c | d |
And the ways
| nodes | highway | name |
| ab | primary | road |
| bc | primary | road |
| cd | primary | road |
When I route I should get
| from | to | route | turns |
| a | d | road,road | depart,arrive |
```
Guidance guarantees only essential maneuvers. You will always see `depart` and `arrive` as well as all turns that are not obvious.
So the following scenario does not change the instructions
```
Background:
Given the profile "car"
Given a grid size of 10 meters
Scenario: Testbot - Straight Road
Given the node map
| a | b |
| d | c |
And the ways
| nodes | highway | name |
| ab | primary | road |
| bc | primary | road |
| cd | primary | road |
When I route I should get
| from | to | route | turns |
| a | d | road,road | depart,arrive |
```
but if we modify it to
```
Background:
Given the profile "car"
Given a grid size of 10 meters
Scenario: Testbot - Straight Road
Given the node map
| a | b | e |
| d | c | |
And the ways
| nodes | highway | name |
| ab | primary | road |
| bc | primary | road |
| cd | primary | road |
| be | primary | turn |
When I route I should get
| from | to | route | turns |
| a | d | road,road,road | depart,continue right,arrive |
```
### Prevent Randomness
Some features in OSRM can result in strange experiences during testcases. To prevent some of these issues, follow the guidelines below.
#### Use Waypoints
Using grid nodes as waypoints offers the chance of unwanted side effects.
OSRM converts the grid into a so called edge-based graph.
```
Scenario: Testbot - Intersection
Given the node map
| | e | |
| b | a | d |
| | c | |
And the ways
| nodes | highway | oneway |
| ab | primary | yes |
| ac | primary | yes |
| ad | primary | yes |
| ae | primary | yes |
```
Selecting `a` as a `waypoint` results in four possible starting locations. Which one of the routes `a,b`, `a,c`, `a,d`, or `a,e` is found is pure chance and depends on the order in the static `r-tree`.
To guarantee discovery, use:
```
Scenario: Testbot - Intersection
Given the node map
| | | e | | |
| | | 4 | | |
| b | 1 | a | 3 | d |
| | | 2 | | |
| | | c | | |
And the ways
| nodes | highway | oneway |
| ab | primary | yes |
| ac | primary | yes |
| ad | primary | yes |
| ae | primary | yes |
```
And use `1`,`2`,`3`, and `4` as starting waypoints. The routes `1,b`, `2,c`, `3,d`, and `4,e` can all be discovered.
#### Allow For Small Offsets
Whenever you are independent of the start location (see use waypoints), the waypoint chosen as start/end location can still influence distances/durations.
If you are testing for a duration metric, allow for a tiny offset to ensure a passing test in the presence of rounding/snapping issues.
#### Don't Rely on Alternatives
Alternative route discovery is a random feature in itself. The discovery of routes depends on the contraction order of roads and cannot be assumed successful, ever.