testsuite: better granularity in a couple of tests on TAP support

testsuite: better granularity in a couple of tests on TAP support

* tests/tap-summary-aux.sh: New auxiliary script, filled with code
moved out from ...
* tests/tap-summary.test: ... this test, from which the checks on
colored testsuite have further been moved into ...
* tests/tap-summary-color.test: ... this new test, which in turn
* tests/Makefile.am (tap_other_tests): Add `tap-summary-color.test'.
(EXTRA_DIST): Distribute `tap-summary-aux.sh'
(tap-summary.log): Depend on `tap-summary-aux.sh'.
(tap-summary-color.log): Likewise.