coverage: some more tests on corner cases of TAP support

coverage: some more tests on corner cases of TAP support

* tests/tap-no-spurious.test: Extend checks verifying that a line
matching, say, the regex "^ok[a-zA-Z0-9_]" is *not* considered a
TAP result.
* tests/tap-no-spurious-numbers.test: New test, checking that our
TAP driver doesn't spuriously recognize as TAP result numbers what
is not (even if it seems pretty close).
* tests/tap-negative-numbers.test: New test, checking that our
TAP driver doesn't spuriously recognize negative TAP result
numbers, but correctly interprets them as test descriptions
instead.
* tests/tap-plan-leading-zero.test, tap-numbers-leading-zero.test:
New tests, checking how our driver fares in recognizing numbers
with leading zeroes in TAP results or TAP plans.
* tests/tap-planskip-malformed.test: New test, checking that a
malformed TAP plan is not recognized as a valid plan.
* tests/tap-plan-whitespace.test: New test, checking that a TAP
plan line with trailing whitespace is recognized and handled
correctly.
* tests/Makefile.am (tap_with_common_setup_tests): Update.