bridge tests: Re-enable STP test

This test should no longer provoke large amounts of traffic, which can
overwhelm single-core systems, preventing them from making progress in the
tests.

The test can now be re-enabled.

PR:		246448

tests/sys/net/if_bridge_test.sh

@@ -76,10 +76,6 @@ stp_head()
 
 stp_body()
 {
-	if [ "$(atf_config_get ci false)" = "true" ]; then
-		atf_skip "https://bugs.freebsd.org/246448"
-	fi
-
 	vnet_init
 
 	epair_one=$(vnet_mkepair)