Fix some issues with TAP tests for postgres -C

Fix some issues with TAP tests for postgres -C

This addresses two issues with the tests added in 0c39c292 for runtime
GUCs:
- Re-enable the test on Msys.  The test could fail because of \r\n
generated by Msys perl.  0d91c52a has taken care of this issue.
- Allow the test to run in the context of a privileged account.  CIs
running under privileged accounts would fail on permission failures, as
reported by Andres Freund.  This issue is fixed by wrapping the postgres
command within pg_ctl as the latter will take care of any permissions
needed.  The test checking a failure of postgres -C for a runtime
parameter with an instance running is removed, as pg_ctl produces an
unstable error code (no need for a CI to reproduce that).

Discussion: https://postgr.es/m/20210921032040.lyl4lcax37aedx2x@alap3.anarazel.de