test: skip some tst_qtextscriptengine tests when fonts are missing

test: skip some tst_qtextscriptengine tests when fonts are missing

This autotest may fail depending on the version of fonts which are
installed on the system.  It has no way to verify if the correct fonts
are installed, and it's unclear from where the correct version of the
fonts may be obtained.

Therefore, disable the test by default unless the user has indicated
that they have a correct setup by setting QT_HAVE_TEST_FONTS=1.

Task-number: QTBUG-20682
Change-Id: I24174520f54af6a9b2c13c9c1347f46555bce3d4
Reviewed-on: http://codereview.qt.nokia.com/2469
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>