Combine initdb tests that successfully create a data directory.

Combine initdb tests that successfully create a data directory.

This eliminates many seconds of test duration and the cause to invoke
"rm -rf", which is typically unavailable on Windows.

This is a backpatch of 1a629c1 which has never been applied to
REL9_4_STABLE.  Per complain from buildarm member drongo.

Reported-by: Tom Lane
Author: Michael Paquier, Noah Misch
Discussion: https://postgr.es/m/11800.1577060649@sss.pgh.pa.us