test: fixed tst_qimagereader and removed CONFIG+=insignificant_test

test: fixed tst_qimagereader and removed CONFIG+=insignificant_test

This test assumed that Qt's available image formats could be determined
at compile time.  This was never correct since the image formats are
loaded from plugins at runtime; in Qt5 it became a real problem since
one image format (svg) was moved out of qtbase into a separate module,
turning this into a circular dependency: this test in qtbase depends on
qtsvg, which depends on qtbase.

Always check the image formats at runtime instead.

Change-Id: I5e770c5b11276c39910e34f232a2fea0a7abaa8c
Reviewed-on: http://codereview.qt.nokia.com/2457
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>