Bump testtools version to support message parameter in assertIn

Commit aa67a51299faa5e51da2e96fcf3b0ba3fc2ddc86 introduced usage of the
`message` parameter to `assertIn`.

The current minimium version of `testtools` doesn't support it, so we need to
bump up to at least version 0.3.6.

Change-Id: I6985bd1daa373348f2f37e26fb5597ac3e46f316
Closes-Bug: 1387870
Merge "Bump testtools version to support message parameter in assertIn"