Fix WikitextContentHandlerTest::testMakeRedirectContent() when content language is not english

Clear the MagicWord object cache, otherwise the magic word names can be cached
in the wrong language, thus making the test fail.

Change-Id: Ia6da94ca2c4f9ca892c81b4aa7cf96108b42ccd5
Merge "Fix WikitextContentHandlerTest::testMakeRedirectContent() when content language is not english"