Further harden tests that might use not-so-compatible tar versions.

Further harden tests that might use not-so-compatible tar versions.

Buildfarm testing shows that OpenSUSE (and perhaps related platforms?)
configures GNU tar in such a way that it'll archive sparse WAL files
by default, thus triggering the pax-extension detection code added by
bc30c704a.  Thus, we need something similar to 852de579a but for
GNU tar's option set.  "--format=ustar" seems to do the trick.

Moreover, the buildfarm shows that pg_verifybackup's 003_corruption.pl
test script is also triggering creation of pax-format tar files on
that platform.  We had not noticed because those test cases all fail
(intentionally) before getting to the point of trying to verify WAL
data.

Since that means two TAP scripts need this option-selection logic, and
plausibly more will do so in future, factor it out into a subroutine
in Test::Utils.  We also need to back-patch the 003_corruption.pl fix
into v18, where it's also failing.

While at it, clean up some places where guards for $tar being empty
or undefined were incomplete or even outright backwards.  Presumably,
we missed noticing because the set of machines that run TAP tests
and don't have tar installed is empty.  But if we're going to try
to handle that scenario, we should do it correctly.

Reported-by: Tomas Vondra <tomas@vondra.me>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/02770bea-b3f3-4015-8a43-443ae345379c@vondra.me
Backpatch-through: 18