qmltest::tst_grabImage::test_equals() is flaky on linux

********* Start testing of qmltest *********
Config: Using QtTest library 5.6.2, Qt 5.6.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 4.9.1 20140922 (Red Hat 4.9.1-10))
...
PASS   : qmltest::tst_grabImage::initTestCase()
FAIL!  : qmltest::tst_grabImage::test_equals() 'verify()' returned FALSE. ()
   Loc: [/home/qt/work/qt/qtdeclarative/tests/auto/qmltest/selftests/tst_grabImage.qml(53)]
PASS   : qmltest::tst_grabImage::cleanupTestCase()
...
PASS   : qmltest::tst_grabImage::initTestCase()
PASS   : qmltest::tst_grabImage::test_equals()
PASS   : qmltest::tst_grabImage::cleanupTestCase()
...
********* Finished testing of qmltest *********