Test case for the locking stuff.

Probably not particularly great at finding issues on all platforms.
Notably Windows doesn't seem to crash without it anyway.

tests/hazmat/backends/test_openssl.py

@@ -11,6 +11,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import threading
+
 import pytest
 
 from cryptography import utils
@@ -97,3 +99,28 @@ def test_handle_unknown_error(self):
                 backend.lib.EVP_F_EVP_DECRYPTFINAL_EX,
                 0
             )
+
+    def test_threads(self):
+        b = Backend()
+
+        def randloop():
+            s = b.ffi.new("char[]", 16)
+            sb = b.ffi.buffer(s)
+            sb[:] = b"\0" * 16
+
+            for i in range(300000):
+                b.lib.RAND_seed(s, 16)
+
+        threads = []
+        for x in range(3):
+            t = threading.Thread(target=randloop)
+            t.daemon = True
+            t.start()
+
+            threads.append(t)
+
+        while threads:
+            for t in threads:
+                t.join(0.1)
+                if not t.isAlive():
+                    threads.remove(t)