Change skipped test error code from 99 to 77

[Ericson2314]

Motivation

Meson uses a venerable GNU convention described in https://www.gnu.org/software/automake/manual/html_node/Scripts_002dbased-Testsuites.html in which:

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.

77 is thus what we want, not 99.

Context

#2503

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