docs: document custom test drivers and protocols

docs: document custom test drivers and protocols

* doc/automake.texi (Simple Tests): Note that the TESTS_ENVIRONMENT
use suggested here is not portable to 'parallel-tests'.
(Simple Tests using parallel-tests): Document new restrictions on
the uses of TESTS_ENVIRONMENT and AM_TESTS_ENVIRONMENT.
(Custom Test Drivers): New section and node.
(Overview of Custom Test Drivers Support): New subsection.
(Declaring Custom Test Drivers in @file{Makefile.am}): Likewise.
(APIs for Custom Test Drivers): Likewise.
(Options): Update description of color-tests.
* lib/am/check ($(TEST_SUITE_LOG)): Remove comments that have been
moved into the manual.
(recheck, recheck-html): Minor adjustments to better conform to the
documentation (this should cause no semantic changes w.r.t. the
former behaviour); minor improvements and extensions to existing
comments.
* tests/test-driver-create-log-dir.test: New test.
* tests/test-driver-strip-vpath.test: Likewise.
* tests/test-driver-global-log.test: Likewise.
* tests/test-driver-recheck.test: Likewise.
* tests/Makefile.am (TESTS): Update.