Make it possible to override test timeout value

Currently, we are setting the test timeout value globally
in .testr.conf. It could be useful to be able to override
it per test case (e.g. for migrations tests which usually
take much longer to run than any other tests).

Related-Bug: #1216851

Change-Id: Icee09609f76f4439f6d981641be521d8d695a77e
Merge "Make it possible to override test timeout value"