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/CMakeLists.txt

@@ -37,7 +37,7 @@ function(add_fmt_test name)
       set(PEDANTIC_COMPILE_FLAGS ${PEDANTIC_COMPILE_FLAGS} -Wno-weak-vtables)
     endif ()
   elseif (ADD_FMT_TEST_MODULE)
-    set(libs gtest test-module)
+    set(libs test-main test-module)
     set_source_files_properties(${name}.cc PROPERTIES OBJECT_DEPENDS test-module)
   else ()
     set(libs test-main fmt)