test: use unique output file names across tests

Without the change tests ran in parallel occasionally clobber outputs of
one another and fail as:

    $ make check -j16 VERBOSE=y
    ...
    FAIL: huge
    ==========
    Converting
    File ../test/data/5.plist is open
    PList XML parsing succeeded
    PList BIN writing succeeded
    PList BIN parsing succeeded
    PList XML writing succeeded
    Size of input and output is different
    Input size : 4292380
    Output size : 4305301
    Comparing
    PList parsing failed
    FAIL huge.test (exit status: 3)

Closes: https://github.com/libimobiledevice/libplist/issues/234#issuecomment-1743820556

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