Make postmaster 003_start_stop.pl test less flaky

Make postmaster 003_start_stop.pl test less flaky

The test is very sensitive to how backends start and exit, because it
tests dead-end backends which occur when all the connection slots are
in use. The test failed occasionally in the CI, when the backend that
was launched for the raw_connect_works() check lingered for a while,
and exited only later during the test. When it exited, it released a
connection slot, when the test expected all the slots to be in use at
that time.

The 002_connection_limits.pl test had a similar issue: if the backend
launched for safe_psql() in the test initialization lingers around, it
uses up a connection slot during the test, messing up the test's
connection counting. I haven't seen that in the CI, but when I added a
"sleep(1);" to proc_exit(), the test failed.

To make the tests more robust, restart the server to ensure that the
lingering backends doesn't interfere with the later test steps.

In the passing, fix a bogus test name.

Report and analysis by Jelte Fennema-Nio, Andres Freund, Thomas Munro.

Discussion: https://www.postgresql.org/message-id/CAGECzQSU2iGuocuP+fmu89hmBmR3tb-TNyYKjCcL2M_zTCkAFw@mail.gmail.com
Backpatch-through: 18