Recent pg_regress changes break testing under SELinux

Who decided this part of pg_regress.c was a good idea?

/* try to create the test tablespace dir if it doesn't exist */
snprintf(testtablespace, MAXPGPATH, "%s/testtablespace", abs_builddir);
if (directory_exists(testtablespace))
rmtree(testtablespace, true);
make_directory(testtablespace);

The regression test Makefile is responsible for preparing that
directory, not pg_regress. This is important because in the Postgres
RPMs, we have to be careful to appease SELinux:

# The tests command the server to write into testtablespace and results.
# On a SELinux-enabled system this will fail unless we mark those directories
# as writable by the server.
cleandirs:
-rm -rf testtablespace results
mkdir testtablespace results
[ -x /usr/bin/chcon ] && /usr/bin/chcon -u system_u -r object_r -t postgresql_db_t testtablespace results

By blowing away the directory and remaking it, pg_regress causes this
carefully-applied labeling to be lost.

As far as I can see the rmtree/make_directory thrashing is useless, and
I'm going to remove it unless a pretty good counter-argument is made.

regards, tom lane