tap/awk: correctly extract message from SKIP plans

tap/awk: correctly extract message from SKIP plans

* lib/tap-driver.sh (/^1\.\.0+[ \t]*#/): Add a forgotten `?' to
the regex used to extract the skip message from a "1..0" TAP plan.
* tests/planskip.test: Check for the just-fixed bug.
* tests/tap-planskip-unplanned-corner.test: Don't be too strict
w.r.t. the formatting of the "skip whole test" message, to avoid
failing due to bugs irrelevant for this test (like the one just
fixed).