pgsql: Tighten TAP tests' tracking of postmaster state some more.

Tighten TAP tests' tracking of postmaster state some more.

Commits 6c4a8903b et al. had a couple of deficiencies:

* The logic I added to Cluster::start to see if a PID file is present
could be fooled by a stale PID file left over from a previous
postmaster.  To fix, if we're not sure whether we expect to find a
running postmaster or not, validate the PID using "kill 0".

* 017_shm.pl has a loop in which it just issues repeated Cluster::start
calls; this will fail if some invocation fails but leaves self->_pid
set.  Per buildfarm results, the above fix is not enough to make this
safe: we might have "validated" a PID for a postmaster that exits
immediately after we look.  Hence, match each failed start call with
a stop call that will get us back to the self->_pid == undef state.
Add a fail_ok option to Cluster::stop to make this work.

Discussion: https://postgr.es/m/CA+hUKGKV6fOHvfiPt8=dOKzvswjAyLoFoJF1iQXMNpi7+hD1JQ@mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cf680bd6538a5a05020ab4d193b43563e9eb0c5e

Modified Files
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src/test/perl/PostgresNode.pm  | 40 ++++++++++++++++++++++++++++++++++------
src/test/recovery/t/017_shm.pl |  3 +++
2 files changed, 37 insertions(+), 6 deletions(-)