[ChromeDriver] Disable flaky testGetSessions

SessionHandlingTest.testGetSessions appears to trigger some race
conditions and is flaky. Disabling it until a fix can be found.

TBR=zmin@chromium.org

Bug: 899919
Change-Id: I3c532559477a3597f20984783eb36acf6355ad7b
Reviewed-on: https://chromium-review.googlesource.com/c/1308657Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604006}

chrome/test/chromedriver/test/run_py_tests.py

@@ -181,6 +181,8 @@ _INTEGRATION_NEGATIVE_FILTER = [
     # RemoteBrowserTest requires extra setup. TODO(johnchen@chromium.org):
     # Modify the test so it runs correctly as isolated test.
     'RemoteBrowserTest.*',
+    # Flaky: https://crbug.com/899919
+    'SessionHandlingTest.testGetSessions',
 ]