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.