test: make bug-reporting message on test failure more explicit.

test: make bug-reporting message on test failure more explicit.

From https://bugs.gnu.org/68855.

* NEWS: mention this.
* lib/am/check.am ($(TEST_SUITE_LOG)) <please_report>: new fn,
new wording.
* t/ax/test-defs.in (failure_footer_text): new wording.
(failure_footer_text_colorized): add red.
* t/ax/tap-summary-aux.sh (failure_footer): use failure_footer_text*.
* t/tap-doc2.sh: change grep.
* t/testsuite-summary-color.sh: likewise.
* t/testsuite-summary-count-many.sh: likewise.
* t/testsuite-summary-count.sh: likewise.
* t/testsuite-summary-reference-log.sh: likewise.