move core regress tests under regress/core

[strk]

This is to allow running core-only tests w/out necessarely _also_ running sfcgal tests, which is currently impossible with a "make check" call (there's no isolated directory for them).

My proposal is to have:

regress/core regress/sfcgal

Then the Makefile in regress/ would descend in both if sfcgal is enabled and only in core otherwise. Users willing to only run core tests can do:

make -C regress/core check

This is particularly useful for running upgrade tests (see utils/check_all_upgrades) when sfcgal was not available in source release.

[strk]

btw, I just noticed that the —upgrade switch is passed in regress/Makefile too, so we're already doing extra work there (maybe this could be avoided by skipping the second call _iff_ —upgrade was already in the env-provided flags

[strk]

In 16929:

Split core and sfcgal tests allowing to run one w/out the other

Closes #4200