capsicum-test: skip SCTP tests if SCTP not available

Skip the test rather than failing if SCTP is not available (e.g., if
sctp.ko is not loaded).

Also submitted upstream as
https://github.com/google/capsicum-test/pull/61

Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46999

(cherry picked from commit 1d83090d85)

contrib/capsicum-test/sctp.cc

@@ -61,6 +61,9 @@ static int SctpClient(int port, unsigned char byte) {
 
 TEST(Sctp, Socket) {
   int sock = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);
+  if (sock == -1 && errno == EPROTONOSUPPORT) {
+    GTEST_SKIP() << "socket(..., IPPROTO_SCTP) -> EPROTONOSUPPORT";
+  }
   EXPECT_OK(sock);
   if (sock < 0) return;