Add a couple of missed .gitignore entries.

Any subdirectory that's ignoring /output_iso/ should also
ignore /tmp_check_iso/, which could be left behind by a
failed pg_isolation_regress_check run.

I think these have been wrong for awhile, but it doesn't
seem important to fix in back branches.

src/test/modules/brin/.gitignore

 # Generated subdirectories
 /output_iso/
 /tmp_check/
+/tmp_check_iso/

src/test/modules/snapshot_too_old/.gitignore

+# Generated subdirectories
 /output_iso/
+/tmp_check_iso/