tap: a "plan with SKIP" after test results is an error

tap: a "plan with SKIP" after test results is an error

This new behaviour is both more natural and more consistent with
that of the `prove' utility.

* lib/tap-driver (handle_tap_plan): Do not stop TAP parsing when
a "plan with SKIP" line (e.g., "1..0 # SKIP") is encountered, and
do not print a "SKIP" line if some TAP result has already been
seen.
* tests/tap-skip-whole.test: Adapt.
* tests/tap-skip-whole-lastline.test: Likewise.
* tests/tap-global-result.test: Adapt and extend.
* tests/tap-skip-plan-errors.test: Likewise, and fix an obsolete
small part of a comment.
* tests/tap-skip-whole-bailout.test: New test.
* tests/tap-skip-whole-unplanned.test: Likewise.
* tests/tap-skip-whole-continue.test: Likewise.
* tests/Makefile.am (tap_with_common_setup_tests): Update.