Avoid using internal testlib API in QDbusConnection autotest.

Avoid using internal testlib API in QDbusConnection autotest.

QCOMPARE should only be used in a test function because it makes the
test function return if the compare fails.  The test wants to compare
without returning on failure because the compare is inside a helper
function called by many test functions, so the test was calling
testlib's internal QTest::compare_helper() functions instead of
QCOMPARE.

This commit makes this code slightly less objectionable by calling the
public QTest::qCompare() instead.

Change-Id: Ida17a641e89f8a297d6a036449f44b33aa266368
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>