test: Fix XPASS for tst_QScriptJsTestSuite::ecma/Math(Math.pow()

This tests seem to be passing on MinGW 4.8, so removing them
from the expect_fail.txt file.

Change-Id: I3786d410f1e0937ddc1ceedd8ffd3c07d2619c40
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>