Revert "Remove reset of testtablespace from pg_regress on Windows"

This reverts commit 2b2a070, that moved the reset of path
"testtablespace" used by the regression tests as a path for tablespaces
(via --outputdir) from pg_regress to the MSVC script vcregress.pl, as
this broke the behavior added by ce5d342 to be able to safely run the
regression test suite with an administrative Windows account using a
restricted token.

Note that before 2b2a070, the code doing the reset in pg_regress.c
included a comment telling that we had better move that out to a
different place, leading to the mistake done in 2b2a070.  Fix this
comment, and document instead that we had better never remove this code,
for the sake of not breaking again the behavior we expect on Windows.

Thanks to Thomas Munro and Andrew Dunstan for the discussion.

Discussion: https://postgr.es/m/6d9eee97-54c8-e14a-48f7-3194e712f54f@2ndQuadrant.com
Discussion: https://postgr.es/m/CA+hUKGLiieEzfrdWxWFE+_wnXho_F5Smx972X1wEubhS7v1q9g@mail.gmail.com
