test: Fix actual vs. expected in virtTestCompareFiles

test: Fix actual vs. expected in virtTestCompareFiles

Commit ca329299 added a utility function virtTestCompareFiles() to
eliminate repetitive code in several test programs. It unfortunately
calls virtTestDifference() with the arguments in the wrong order -
strcontent is the "actual" output gathered by the test rig, while
filecontent is the "expected", and virtTestDifference() wants expected
(filecontent) followed by actual (strcontent), but
virtTestCompareFiles() does the opposite, which can make the output a
bit confusing when there is a failure.