test_replaygain: fix complicated and incorrect exception handling
test_replaygain: fix complicated and incorrect exception handling

This is an incorrect translation of a python 2 reraise to python 3.
With python 3, however, we can just rely on exception chaining to get
the traceback, so get rid of the complicated re-raising entirely, with
the additional benefit that the exception from the tear-down is also
shown.

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