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.

src/test/modules/brin/Makefile

 # src/test/modules/brin/Makefile
 
-EXTRA_CLEAN = ./isolation_output
+# Note: because we don't tell the Makefile there are any regression tests,
+# we have to clean those result files explicitly
+EXTRA_CLEAN = $(pg_regress_clean_files) ./isolation_output
+
 EXTRA_INSTALL=contrib/pageinspect
 
 ISOLATIONCHECKS=summarization-and-inprogress-insertion