Make ibus-config tests side effect free
Make ibus-config tests side effect free

Previously we assumed that test cases are executed in the order of
registration.  This is no longer true after glib change 89aa9dbd which
fixed test case grouping.  Clear the default match rule in each test
case to avoid side effect.

BUG=

Review URL: https://codereview.appspot.com/10069045

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