Avoid picking already-bound TCP ports in kerberos and ldap test suites.

src/test/kerberos and src/test/ldap need to run a private authentication
server of the relevant type, for which they need a free TCP port.
They were just picking a random port number in 48K-64K, which works
except when something's already using the particular port.  Notably,
the probability of failure rises dramatically if one simply runs those
tests in a tight loop, because each test cycle leaves behind a bunch of
high ports that are transiently in TIME_WAIT state.

To fix, split out the code that PostgresNode.pm already had for
identifying a free TCP port number, so that it can be invoked to choose
a port for the KDC or LDAP server.  This isn't 100% bulletproof, since
conceivably something else on the machine could grab the port between
the time we check and the time we actually start the server.  But that's
a pretty short window, so in practice this should be good enough.

Back-patch to v11 where these test suites were added.

Patch by me, reviewed by Andrew Dunstan.

Discussion: https://postgr.es/m/3397.1564872168@sss.pgh.pa.us