xmlpatternsdiagnosticsts test: use correct baseline

At least on OS X, the test would use the baseline of xmlpatternsxqts
since that's where tst_suitetest.h is. QFINDTESTDATA would then resolve
Baseline.xml relative to that.

Change-Id: I860d7e3143365b33901f721caed813d4d594f28d
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>