gm .check should work in win

currently it cannot call run-tests.py since it use Linux path

Change-Id: I15af9c7e6503e6d473611a24f5f223ff68b1dbbd
Reviewed-on: https://chromium-review.googlesource.com/c/1484110Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#59846}

tools/dev/gm.py

@@ -294,7 +294,7 @@ class Config(object):
       tests = ""
     else:
       tests = " ".join(self.tests)
-    return _Call("tools/run-tests.py --outdir=%s %s" %
+    return _Call(os.path.join("tools", "run-tests.py") + " --outdir=%s %s" %
                    (GetPath(self.arch, self.mode), tests))
 
 def GetTestBinary(argstring):