tests: avoid spurious failure in 'parallel-tests3.test'

tests: avoid spurious failure in 'parallel-tests3.test'

This fixes automake bug#8788.

* tests/parallel-tests3.test: To ensure that the serial run of
the dummy testsuite is still ongoing when the parallel run has
terminated, use `kill -0', not a bare `kill'.  This will prevent
a testsuite crash on NetBSD 5.1, and a testsuite hang on FreeBSD
8.2.  Also, since we are at it, try harder to avoid possible
hangs of the script in other unusual situations.