Set global $wgContLang for CopyrightMessageBuilderTest

this test is failing on sometimes on travis, for non-english wikis,
due to not controlling for the global variable.

Change-Id: I8acdb23db7e1fcb3a284c6ddc6f70d182981b0d3
Merge "Set global $wgContLang for CopyrightMessageBuilderTest"