lib: propagate test status 'skip'

[ligurio]

Patch add support for a SKIP directive supported by a TAP13 specification [1]. When all testcases have SKIP directive the overall test status in test-run will have 'skip' test status.

TAP version 13
1..3
ok 1 # SKIP
ok 2 # SKIP
ok 3 # SKIP

TAP13 specification supports setting SKIP directive in a test plan:

TAP version 13
1..0 # SKIP

This is not supported by test-run.

1. https://testanything.org/tap-version-13-specification.html

Closes #381