tests: use 'parallel-tests' Automake option by default

tests: use 'parallel-tests' Automake option by default

This will help our transition from 'serial-tests' to 'parallel-tests'
as the default test suite driver enabled by a TESTS assignment in the
input Makefile.am.  Note that that change of default will only take
place in master, though.

* defs: Pass the 'parallel-tests' option to the AM_INIT_AUTOMAKE
invocation in the created 'configure.ac' stub, unless the variable
'am_serial_tests' is set to "yes".  Don't pay attention anymore to
the 'am_parallel_tests' variable, that's obsolete now.
* defs-static.in: Warn if the 'am_serial_tests' variable is set in the
environment; conversely, don't warn anymore about 'am_parallel_tests'
being set in the environment.
* Makefile.am (AM_TESTS_ENVIRONMENT): Nullify the 'am_serial_tests'
variable instead of the now-obsolete 'am_parallel_tests' one.
* syntax-checks.mk (sc_tests_obsolete_variables): Also warn against
uses of 'am_parallel_tests', which is now deprecated in favor of
'am_serial_tests'.  Similarly, if a use of 'parallel_tests' is seen,
suggest using 'am_serial_tests' instead, not 'am_parallel_tests'.
* gen-testsuite-part: Now that we use the 'parallel-tests' by default
in our tests, we need to completely change the logic and semantics of
generation of sibling tests for those tests that check the Automake
generated testsuite harness itself.  Do that, and give a complete
explanation of the new logic and semantics in the relevant comments.
* t/README: Update.
* Lots of test cases: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>