tests: add some self-documentation to tests
tests: add some self-documentation to tests

Alon tried './qemuxml2argvtest --help' to figure out a test failure,
but it didn't help.  The information is in HACKING, but it doesn't
hurt to make the tests also provide their own help.

Signed-off-by: Alon Levy <816c907b754fa7b5d4bb5eb91e0fc01e5510e64d@redhat.com>
Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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