tests: Adjust testsuite return codes to automake guidelines

As per the manual[1]:

"When no test protocol is in use, an exit status of 0 from a test script
will denote a success, an exit status of 77 a skipped test, an exit
status of 99 a hard error, and any other exit status will denote a
failure."

[1] https://www.gnu.org/software/automake/manual/html_node/Scripts_002dbased-Testsuites.html

Signed-off-by: Phil Sutter <phil@nwl.cc>

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