Include test sources to pick up functions and classes from the module rather than from the non-modular library which is baked into the test-main library. (#2356)

This averts linker problems:
 - strong ownership model: missing linker symbols
 - weak ownership model: duplicate linker symbols

Simplify `gtest-extra.cc` while at it.

test/CMakeLists.txt

@@ -41,7 +41,7 @@ function(add_fmt_test name)
       set(PEDANTIC_COMPILE_FLAGS ${PEDANTIC_COMPILE_FLAGS} -Wno-weak-vtables)
     endif ()
   elseif (ADD_FMT_TEST_MODULE)
-    set(libs test-main test-module)
+    set(libs gtest test-module)
     set_source_files_properties(${name}.cc PROPERTIES OBJECT_DEPENDS test-module)
   else ()
     set(libs test-main fmt)
@@ -97,7 +97,7 @@ if (FMT_CAN_MODULE)
     $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>)
   enable_module(test-module)
 
-  add_fmt_test(module-test MODULE)
+  add_fmt_test(module-test MODULE test-main.cc)
   if (MSVC)
     target_compile_options(test-module PRIVATE /utf-8 /Zc:__cplusplus
                            /Zc:externConstexpr /Zc:inline)