docs: rework and extend documentation on testsuites support

docs: rework and extend documentation on testsuites support

* doc/automake.texi (Generalities about Testing): New section.
(Simple Tests using parallel-tests): Section removed, merged
into ...
(Simple Tests): ... this one, which on the other hand has been
subdivided into ...
(Script-based Testsuites, Serial Test Harness, Parallel Test
Harness): ... these new subsection.
(DejaGnu Tests): Minor adjustments.
Other related typofixes and rewordings throughout the manual;
in particular, avoid to use the term "test driver" for three
different concepts (and use instead "test harness" and "test
runner" where appropriate).
* tests/tap-doc.test: New test.
* tests/tap-no-disable-hard-error.test: Likewise.
* tests/Makefile.am (TESTS): Update.