Avoid generation of `tests/defs-p' file

Avoid generation of `tests/defs-p' file

* tests/defs.in: In the generated `configure.in' snippet: call
`AM_INIT_AUTOMAKE' with the `parallel-tests' option if the shell
variable `parallel_tests' is set to `yes'.
* tests/Makefile.am (defs-p): Target removed.
(check_SCRIPTS): Removed `defs-p'.
(clean-local-check): Do not unlink `defs-p' anymore.
($(parallel_tests)): Transformation rules for the test scripts
adjusted.
* tests/gen-parallel-tests: Selection rules for the test
scripts adjusted.
* tests/parallel-tests.test: Set `$parallel_tests' to `yes'
then include `./defs' (rather than simply including `./defs-p').
* tests/parallel-tests2.test: Likewise.
* tests/parallel-tests3.test: Likewise.
* tests/parallel-tests4.test: Likewise.
* tests/parallel-tests5.test: Likewise.
* tests/parallel-tests6.test: Likewise.
* tests/parallel-tests7.test: Likewise.
* tests/parallel-tests8.test: Likewise.
* tests/parallel-tests9.test: Likewise.
* tests/parallel-tests10.test: Likewise.
* tests/README (Section "Writing Test Cases", subsection "Do"):
Adjusted the parts referring to tests checking `parallel-tests'
behaviour.  Some other minor related improvements.
* tests/.gitignore (defs-p): Removed.