Moving this logic into pg_regress fixes a potential failure with
parallel tests when pg_upgrade and the main regression test suite both
trigger the makefile rule that cleaned up testtablespace/ under
src/test/regress.  Even if pg_upgrade was triggering this rule, it has
no need to do so as it uses a different tablespace path.  So if
pg_upgrade triggered the makefile rule for the tablespace setup while
the main regression test suite ran the tablespace cases, it would fail.

61be85af was a similar attempt at achieving that, but that broke cases
where the regression tests require to run under an Administrator
account, like with Appveyor.

Reported-by: Andres Freund, Kyotaro Horiguchi
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/20201209012911.uk4d6nxcnkp7ehrx@alap3.anarazel.de