Skip ForkDetect tests on iOS

iOS cannot actually fork. Since they have pthread_atfork, we set that up
anyway just in case, but since fork will just fail, we should skip the
tests.

Change-Id: I05f34e13fae84f65f950cd0d71e238ed0366b98e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63765
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/rand/fork_detect_test.cc

diff --git a/crypto/fipsmodule/rand/fork_detect_test.cc b/crypto/fipsmodule/rand/fork_detect_test.cc
index f51530a..f9cde28 100644
--- a/crypto/fipsmodule/rand/fork_detect_test.cc
+++ b/crypto/fipsmodule/rand/fork_detect_test.cc
@@ -18,7 +18,8 @@
 
 // TSAN cannot cope with this test and complains that "starting new threads
 // after multi-threaded fork is not supported".
-#if defined(OPENSSL_FORK_DETECTION) && !defined(OPENSSL_TSAN)
+#if defined(OPENSSL_FORK_DETECTION) && !defined(OPENSSL_TSAN) && \
+    !defined(OPENSSL_IOS)
 #include <errno.h>
 #include <inttypes.h>
 #include <stdio.h>
@@ -173,4 +174,4 @@
   EXPECT_EQ(start, CRYPTO_get_fork_generation());
 }
 
-#endif  // OPENSSL_FORK_DETECTION && !OPENSSL_TSAN
+#endif  // OPENSSL_FORK_DETECTION && !OPENSSL_TSAN && !OPENSSL_IOS