Add basic facades for Extractor and Contractor run methods.

Based on idea suggested in comments to #3776, simplifies
use of extractor and contractor as libraries.

unit_tests/library/contract.cpp

@@ -0,0 +1,21 @@
+#include <boost/test/test_case_template.hpp>
+#include <boost/test/unit_test.hpp>
+
+#include "osrm/contractor.hpp"
+#include "osrm/contractor_config.hpp"
+
+#include <tbb/task_scheduler_init.h> // default_num_threads
+
+BOOST_AUTO_TEST_SUITE(library_contract)
+
+BOOST_AUTO_TEST_CASE(test_contract_with_invalid_config)
+{
+    using namespace osrm;
+
+    osrm::ContractorConfig config;
+    config.requested_num_threads = tbb::task_scheduler_init::default_num_threads();
+    BOOST_CHECK_THROW(osrm::contract(config),
+                      std::exception); // including osrm::util::exception, etc.
+}
+
+BOOST_AUTO_TEST_SUITE_END()

unit_tests/library/extract.cpp

@@ -0,0 +1,27 @@
+#include <boost/test/test_case_template.hpp>
+#include <boost/test/unit_test.hpp>
+
+#include "osrm/extractor.hpp"
+#include "osrm/extractor_config.hpp"
+
+#include <tbb/task_scheduler_init.h> // default_num_threads
+
+BOOST_AUTO_TEST_SUITE(library_extract)
+
+BOOST_AUTO_TEST_CASE(test_extract_with_invalid_config)
+{
+    osrm::ExtractorConfig config;
+    config.requested_num_threads = tbb::task_scheduler_init::default_num_threads();
+    BOOST_CHECK_THROW(osrm::extract(config),
+                      std::exception); // including osrm::util::exception, osmium::io_error, etc.
+}
+
+BOOST_AUTO_TEST_CASE(test_extract_with_valid_config)
+{
+    osrm::ExtractorConfig config;
+    config.input_path = {OSRM_TEST_DATA_DIR "/monaco.osm.pbf"};
+    config.requested_num_threads = tbb::task_scheduler_init::default_num_threads();
+    BOOST_CHECK_NO_THROW(osrm::extract(config));
+}
+
+BOOST_AUTO_TEST_SUITE_END()