Fix build against Qt 5.9

Somehow it's the case that QtTest 5.9 doesn't know how to QCOMPARE a QString
with a normal string literal:

  ld: error: undefined symbol: bool QTest::qCompare<QString, char [31]>(QString
  const&, char const (&) [31], char const*, char const*, char const*, int)

Change-Id: I13ce50689d9f1c157378b862fb1ed4a572ac7f79
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>