Move --resources_dir to its right place.

We needed a hack in test_main_lib.cc to ensure fileutils were always
linked with test binaries downstream. When I removed the hack, it
broke the binaries that were _not_ using fileutils because a certain
bazel rule expects to be able to pass the flag to all test binaries.

The solution is to move the flag to test_main_lib.cc. This is the
right place for it since it's apparently in the contract of a WebRTC
test binary to support this flag. We then have to pass the value
down to the override, which is why I add a new function for that.
I leave the flag unimplemented in OSS because no one is using it
here anyway. It will be implemented downstream.

Bug: webrtc:9792
Change-Id: I21b3deb43bf0cd56d6aa2622dc5519370a0307a9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156568
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29474}

test/testsupport/file_utils_override.cc

@@ -150,6 +150,12 @@ std::string ResourcePath(const std::string& name,
 #endif
 }
 
+void OverrideResourcesDir(const std::string& resources_dir) {
+  RTC_CHECK(false)
+      << "Setting the resource dir is not supported in open-source "
+         "tests.";
+}
+
 }  // namespace internal
 }  // namespace test
 }  // namespace webrtc