Tests: skip certain tests on FreeBSD 12.0.

The tests trip over a bug in TCP with small "listen .. sndbuf".
See for details: https://svnweb.freebsd.org/changeset/base/342280.

h2_headers.t

diff --git a/h2_headers.t b/h2_headers.t
index d407c77..185da1b 100644
--- a/h2_headers.t
+++ b/h2_headers.t
@@ -644,6 +644,9 @@
 
 # response header frame sent in parts
 
+SKIP: {
+skip 'broken tcp', 5 if $^O eq 'freebsd' and $Config{osvers} =~ '12.0-release';
+
 $s = Test::Nginx::HTTP2->new(port(8082));
 $s->h2_settings(0, 0x5 => 2**17);
 
@@ -681,6 +684,8 @@
 
 }
 
+}
+
 # max_field_size - header field name
 
 $s = Test::Nginx::HTTP2->new(port(8084));