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/gtest-extra.cc

@@ -32,11 +32,10 @@ output_redirect::~output_redirect() FMT_NOEXCEPT {
 }
 
 void output_redirect::flush() {
-#  if EOF != -1
-#    error "FMT_RETRY assumes return value of -1 indicating failure"
-#  endif
   int result = 0;
-  FMT_RETRY(result, fflush(file_));
+  do {
+    result = fflush(file_);
+  } while (result == EOF && errno == EINTR);
   if (result != 0) throw fmt::system_error(errno, "cannot flush stream");
 }