Avoid creating testtablespace directories where not wanted.

Avoid creating testtablespace directories where not wanted.

Recently we refactored things so that pg_regress makes the
"testtablespace" subdirectory used by the core regression tests,
instead of doing that in the makefiles.  That had the undesirable
side effect of making such a subdirectory in every directory that
has "input" or "output" test files.  Since these subdirectories
remain empty, git doesn't complain about them, but nonetheless
they're clutter.

To fix, invent an explicit --make-testtablespace-dir switch,
so that pg_regress only makes the subdirectory when explicitly
told to.

Discussion: https://postgr.es/m/2854388.1621284789@sss.pgh.pa.us