testsuite: avoid re-running few tests with 'parallel-tests' option

testsuite: avoid re-running few tests with 'parallel-tests' option

Some tests in our testsuite use the 'simple-tests' driver only
marginally, or simply as a mean to conveniently check unrelated
invariants.  It makes little sense to force these tests to also
run with the 'parallel-tests' Automake option active, as doing so
offers no real gain in coverage, while often causing a measurable
overhead in execution time (for an already too-slow testsuite).

* tests/pr401.test (parallel_tests): Define to "no", to prevent
the generation of a sibling test script using the 'parallel-tests'
driver.
* tests/pr401b.test: Likewise.
* tests/pr401c.test: Likewise.