Unit tests

Valhalla currently uses googletest for unit testing.

• The basics are covered in googletest primer
• More advanced topics are covered here

Important things to note: * EXPECT_XXX macros mark the test as failed but continue execution * Allows to test more at once * A preferred way over ASSERT_XXX * ASSERT_XXX family should be used when it's pointless to continue the test * e.g. the method we test returns nullptr instead of a valid pointer * Tests execution order is unspecified * If the order is needed fixtures or test suite environments may help * Sometimes it may hint the test design needs rethinking * Be careful with _ in test names - gtest joins a suite name with test name using _ * TEST(Foo, Basic_test) and TEST(Foo_Basic, test) will produce a conflict * In practice it never happens - no need to worry much about it * Be aware of curly braces{} inside macros * A typical case -> container with initializer list creation inside assert macro * Preprocessor is dumb and thinks commas separate the arguments * ASSERT_EQ(foo, vector{1,2,3}); * Fixing this can be done by adding () - ASSERT_EQ(foo, (vector{1,2,3})); * Disabling a test is easy - just prefix its name with DISABLED_ * TEST(PredictiveTraffic, DISABLED_test_predictive_traffic) * They are still compiled and there is always a warning each time the suite runs