Reduce dependencies on rtc::FileSystem in FileRotatingStream tests, adding helpers in webrtc::test::

webrtc/test/testsupport/fileutils.cc

Index: webrtc/test/testsupport/fileutils.cc
diff --git a/webrtc/test/testsupport/fileutils.cc b/webrtc/test/testsupport/fileutils.cc
index 899bf31f9070ac5c5a9166655ca3a164c461e110..02ef06580bbcec4203d46ac1c23f42d4e279ebd6 100644
--- a/webrtc/test/testsupport/fileutils.cc
+++ b/webrtc/test/testsupport/fileutils.cc
@@ -232,6 +232,22 @@ bool CreateDir(const std::string& directory_name) {
   return true;
 }
 
+bool RemoveDir(const std::string& directory_name) {
+#ifdef WIN32
+    return RemoveDirectoryA(directory_name.c_str()) != FALSE;
+#else
+    return rmdir(directory_name.c_str()) == 0;
+#endif
+}
+
+bool RemoveFile(const std::string& file_name) {
+#ifdef WIN32
+  return DeleteFileA(file_name.c_str()) != FALSE;
+#else
+  return unlink(file_name.c_str()) == 0;
+#endif
+}
+
 std::string ResourcePath(const std::string& name,
                          const std::string& extension) {
 #if defined(WEBRTC_IOS)