Test: enhance 037.failover_session/test.sh.

Test: enhance 037.failover_session/test.sh.

Previously it mistakenly executed pg_ctl stop after starting pgbench
in background.  The smart shutdown always allows pgbench to run
successfully with pgbench -C option not being set because backend
does not shutdown while client session continues. In order to shutdown
backend in the middle of pgbench run, tweak health check parameters so
that it detects the backend down as soon as possible. This will
trigger failover in the middle of pgbench run.

With these changes Pgpool-II 4.5 and beyond succeeds in all 4 tests,
while pre-4.5 will fail in all 4 tests (that was the originally
expected result).

Also allow to run all the 4 tests even if some tests fail so that we
can check which of which test failed.