Improve module testing (#3397)

* use the standard `test-main.cc` component instead of injected test infrastructure sources
 * undo now obsolete commit `00235d8a` from July 2021
 * Clang cannot import user-defined literals as it seems -> disable test
 * Clang emits duplicate, non-mergeable copies of `detail::buffer`'s vtable, causing linker errors -> disable test

test/gtest-extra.h

@@ -12,12 +12,7 @@
 
 #include <string>
 
-#ifdef FMT_MODULE_TEST
-import fmt;
-#else
-#  include "fmt/os.h"
-#endif  // FMG_MODULE_TEST
-
+#include "fmt/os.h"
 #include "gmock/gmock.h"
 
 #define FMT_TEST_THROW_(statement, expected_exception, expected_message, fail) \