automake: allow omitting logs of skipped tests from test-suite.log.

automake: allow omitting logs of skipped tests from test-suite.log.

* doc/automake.texi (Parallel Test Harness): Describe the contents of
test-suite.log. Mention IGNORE_SKIPPED_LOGS.
(Command-line arguments for test drivers): Document the
--collect-skipped-logs option.
* lib/test-driver (print_usage): Document --collect-skipped-logs.
(collect_skipped_logs): New variable.
(gcopy): Use collect_skipped_logs.
* lib/am/check.am (am__common_driver_flags): Add the contents of
am__collect_skipped_logs.
(am__check_pre): Set am__collect_skipped_logs.
* t/parallel-tests-without-skipped-logs.sh: New file, based on
t/parallel-tests-no-color-in-log.sh.
* t/list-of-tests.mk (handwritten_TESTS): Add it.
* NEWS: Mention the change.