minor regression test refactor

I noticed that the "bigcheck" and "bigtest" targets of
src/test/regress/GNUMakefile don't have the right dependencies:

$ make -C src/test/regress clean bigcheck
[...]
make: ./pg_regress: Command not found
make: *** [bigcheck] Error 127

Attached is a patch that fixes this, by making "bigcheck" and "bigtest"
depend on the "all" target. Along the way, I removed some old Makefile
targets that were just aliases for existing targets, and refactored the
code for setting up the tablespace directory to avoid repeating it five
times.

Barring any objections, I'll apply this tomorrow.

-Neil