Add JoinFilename to testsupport code, replacing use of rtc::Pathname.

This is a partial revert of https://codereview.webrtc.org/2533213005,
deleting rtc::File methods accepting an rtc::Pathname argument.

Bug: webrtc:6424
Change-Id: Ib16bdc7294dbddfa12ba9ae206c024ff97e529a4
Reviewed-on: https://webrtc-review.googlesource.com/80180
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23489}

test/testsupport/fileutils.h

@@ -67,8 +67,10 @@ std::string GenerateTempFilename(const std::string& dir,
 //           If a directory path is prepended to the filename, a subdirectory
 //           hierarchy reflecting that path is assumed to be present.
 //    extension - File extension, without the dot, i.e. "bmp" or "yuv".
-std::string ResourcePath(const std::string& name,
-                         const std::string& extension);
+std::string ResourcePath(const std::string& name, const std::string& extension);
+
+// Joins directory name and file name, separated by the path delimiter.
+std::string JoinFilename(const std::string& dir, const std::string& name);
 
 // Gets the current working directory for the executing program.
 // Returns "./" if for some reason it is not possible to find the working