Convert test_rand to use pytest-style tests (#563)

Fix up the assert helpers, subclass form `object` rather than test case,
and use parametrization where appropriate.

One helper method on the original `TestCase` was the ability to create
temporary directories that were cleaned up at the end of the test --
now we use a pytest fixture instead: http://doc.pytest.org/en/latest/tmpdir.html

Addresses #340.
diff --git a/src/OpenSSL/rand.py b/src/OpenSSL/rand.py
index cce1327..a22f9e5 100644
--- a/src/OpenSSL/rand.py
+++ b/src/OpenSSL/rand.py
@@ -107,7 +107,7 @@
     """
     Check whether the PRNG has been seeded with enough data.
 
-    :return: :obj:`True` if the PRNG is seeded enough, :obj:`False` otherwise.
+    :return: 1 if the PRNG is seeded enough, 0 otherwise.
     """
     return _lib.RAND_status()