[test] rearrange test files so they're not in the fixtures folder

test/src/main.cpp

@@ -0,0 +1,6 @@
+#include <mbgl/test/util.hpp>
+
+GTEST_API_ int main(int argc, char *argv[]) {
+    testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}