Add missing clean step to src/test/modules/brin/Makefile.

Add missing clean step to src/test/modules/brin/Makefile.

I noticed the tmp_check subdirectory wasn't getting cleaned up
after a check-world run.  Apparently pgxs.mk will only do this
for you if you've defined REGRESS.  The only other src/test/modules
Makefile that does not set that is snapshot_too_old, and it
does it like this.