Add zero-termination checks to QString and QByteArray tests

Add zero-termination checks to QString and QByteArray tests

This uses an alternative approach to the testing formerly introduced
in 4ef5a626. Zero-termination tests are injected into all QCOMPARE/QTEST
invocations. This makes such testing more thorough and widespread, and
gets seamlessly extended by future tests.

It also fixes an issue uncovered by the test where using a past-the-end
position with QString::insert(pos, char), could move uninitialized data
and clobber the null-terminator.

Change-Id: I7392580245b419ee65c3ae6f261b6e851d66dd4f
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>