Document that malloc tests require a longer timeout.

I always forget this.

Change-Id: I9fa15cebb6586985ddc48cdbf9d184a49a8bfb02
Reviewed-on: https://boringssl-review.googlesource.com/11402
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

ssl/test/PORTING.md

@@ -98,6 +98,9 @@ enough calls to reach the number, the shim should fail with exit code
 
 See `crypto/test/malloc.cc` for an example malloc implementation.
 
+Note these tests are slow and will hit Go's test timeout. Pass `-timeout 72h` to
+avoid crashing after 10 minutes.
+
 
 ## Example: Running Against NSS