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}
diff --git a/rtc_base/file_unittest.cc b/rtc_base/file_unittest.cc
index a8e39dd..5a72e5d 100644
--- a/rtc_base/file_unittest.cc
+++ b/rtc_base/file_unittest.cc
@@ -163,39 +163,13 @@
   EXPECT_TRUE(VerifyBuffer(out, 2, 0));
 }
 
-TEST_F(FileTest, OpenFromPathname) {
-  {
-    File file = File::Open(Pathname(path_));
-    ASSERT_TRUE(file.IsOpen()) << "Error: " << LastError();
-  }
-
-  {
-    Pathname path(path_);
-    File file = File::Open(path);
-    ASSERT_TRUE(file.IsOpen()) << "Error: " << LastError();
-  }
-}
-
-TEST_F(FileTest, CreateFromPathname) {
-  {
-    File file = File::Create(Pathname(path_));
-    ASSERT_TRUE(file.IsOpen()) << "Error: " << LastError();
-  }
-
-  {
-    Pathname path(path_);
-    File file = File::Create(path);
-    ASSERT_TRUE(file.IsOpen()) << "Error: " << LastError();
-  }
-}
-
 TEST_F(FileTest, ShouldBeAbleToRemoveFile) {
   {
-    File file = File::Open(Pathname(path_));
+    File file = File::Open(path_);
     ASSERT_TRUE(file.IsOpen()) << "Error: " << LastError();
   }
 
-  ASSERT_TRUE(File::Remove(Pathname(path_))) << "Error: " << LastError();
+  ASSERT_TRUE(File::Remove(path_)) << "Error: " << LastError();
 }
 
 }  // namespace rtc