ZTS: Fix 'could not unmount datasets' on Alma 9

Many tests are failing on AlmaLinux 9 because ZTS could not destroy the
pool in cleanup.  This was due to the test running within the mounted
dataset directory (/var/tmp/testdir).  If you change to a different
directory before doing the zpool destroy the failure goes away.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>

tests/zfs-tests/include/libtest.shlib

@@ -1569,7 +1569,13 @@ function destroy_pool #pool
 			# to destroy a pool to fail with EBUSY. We retry a few
 			# times allowing failures before requiring the destroy
 			# to succeed.
+			#
+			# Also, sometimes we're in the directory we're trying
+			# to unmount so to / when we're doing a destroy.
+			OLDPWD="$PWD"
+			cd /
 			log_must_busy zpool destroy -f $pool
+			cd "$OLDPWD"
 
 			[[ -d $mtpt ]] && \
 				log_must rm -rf $mtpt