[test] Make test filters platform-independent.

Use the same path separator for test cases on the cmd-line
on all platforms as in testcase.GetLabel, which is used to
report failures.

BUG=chromium:511215
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1281453003

Cr-Commit-Position: refs/heads/master@{#30045}

tools/run-tests.py

@@ -505,7 +505,7 @@ def Main():
   else:
     args_suites = OrderedDict() # Used as set
     for arg in args:
-      args_suites[arg.split(os.path.sep)[0]] = True
+      args_suites[arg.split('/')[0]] = True
     suite_paths = [ s for s in args_suites if s in suite_paths ]
 
   suites = []

tools/testrunner/local/testsuite.py

@@ -221,7 +221,7 @@ class TestSuite(object):
     globs = []
     exact_matches = []
     for a in args:
-      argpath = a.split(os.path.sep)
+      argpath = a.split('/')
       if argpath[0] != self.name:
         continue
       if len(argpath) == 1 or (len(argpath) == 2 and argpath[1] == '*'):