Add unit tests for ChangeTags::modifyDisplayQuery()

Add unit tests for ChangeTags::modifyDisplayQuery()

The rest of ChangeTags.php should have tests too, but this is a start.

Also fix up ChangeTags::modifyDisplayQuery() to work when strings
are passed for certain parameters, which its documentation claims
it supports but which leads to errors in practice. Normalize
these parameters to arrays.

Change-Id: I2fe61bcb716a369a14a45c2843817a6557d44f7c