presubmit: Don't overwrite cwd when running tests.

Sometimes 'cwd' has already been specified, for example by GetPythonUnitTests.
Don't overwrite it if it has been.

TBR=agable@chromium.org

Bug: 829084
Change-Id: Ide65c2811bcdd2628d4e886227d634dc66bfd297
Reviewed-on: https://chromium-review.googlesource.com/1038665
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

presubmit_support.py

@@ -785,7 +785,8 @@ class InputApi(object):
         tests.append(t)
         if self.verbose:
           t.info = _PresubmitNotifyResult
-        t.kwargs['cwd'] = self.PresubmitLocalPath()
+        if not t.kwargs.get('cwd'):
+          t.kwargs['cwd'] = self.PresubmitLocalPath()
     self.thread_pool.AddTests(tests, parallel)
     if not self.parallel:
       msgs.extend(self.thread_pool.RunAsync())

tests/presubmit_unittest.py

@@ -2785,8 +2785,6 @@ class CannedChecksUnittest(PresubmitTestsBase):
     input_api = self.MockInputApi(change, False)
     input_api.verbose = True
     unit_tests = ['allo', 'bar.py']
-    input_api.PresubmitLocalPath().AndReturn(self.fake_root_dir)
-    input_api.PresubmitLocalPath().AndReturn(self.fake_root_dir)
     cmd = ['bar.py', '--verbose']
     if input_api.platform == 'win32':
       cmd.insert(0, 'vpython.bat')
@@ -2814,7 +2812,6 @@ class CannedChecksUnittest(PresubmitTestsBase):
     input_api.verbose = True
     input_api.logging = self.mox.CreateMock(logging)
     input_api.PresubmitLocalPath().AndReturn(self.fake_root_dir)
-    input_api.PresubmitLocalPath().AndReturn(self.fake_root_dir)
     path = presubmit.os.path.join(self.fake_root_dir, 'random_directory')
     input_api.os_listdir(path).AndReturn(['.', '..', 'a', 'b', 'c'])
     input_api.os_path.isfile = lambda x: not x.endswith('.')