test: mark test flaky on FreeBSD

Mark test-net-socket-local-address flaky on FreeBSD. It times out with
some frequency on CI.

Ref: #2475
PR-URL: #4016
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/parallel/parallel.status

@@ -21,3 +21,4 @@ test-child-process-exit-code         : PASS,FLAKY
 [$system==solaris] # Also applies to SmartOS
 
 [$system==freebsd]
+test-net-socket-local-address     : PASS,FLAKY