test harness: be aware of more metadata, simplify test drivers

test harness: be aware of more metadata, simplify test drivers

* lib/am/check.am ($(TEST_SUITE_LOG)):  When producing the
global test log, take into account the new metadata field
`:global-test-result:, to write nicely formatted RST section
titles, instead of leaving this chore to the individual test
drivers.
(am_rst_section): Re-introduce this variable, as removed in
commit `v1.11-870-ga27c9c4'.
* lib/test-driver, lib/tap-driver: Write the global test result
as metadata in the `.trs' file, not as (part of) a formatted RST
subsection title in the `.log' file.  Related simplifications;
in particular, get rid of temporary files usage.
* doc/automake.texi (Log files generation and test results
recording): Document the new metadata.
* tests/check12.test: Update.
* tests/parallel-tests-harderror.test: Likewise.
* tests/parallel-tests-interrupt.test: Likewise.
* tests/parallel-tests-log-override-2.test: Likewise.
* tests/parallel-tests-log-override-recheck.test: Likewise.
* tests/parallel-tests-unreadable.test: Likewise.
* tests/tap-global-result.test: Likewise.
* tests/test-metadata-results.test: Likewise.
* tests/test-log.test: Likewise, and another minor unrelated fix.
* tests/test-metadata-global-result.test: New test.
* tests/Makefile.am (TESTS): Update.