test: use unique output file names across tests

[trofi]

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