More tests with hard-coded 'random' values.

This reduces noise in the code coverage measurements.
diff --git a/tests/test_key.py b/tests/test_key.py
index 24f6501..833a6aa 100644
--- a/tests/test_key.py
+++ b/tests/test_key.py
@@ -43,7 +43,9 @@
 
     def test_custom_getprime_func(self):
         # List of primes to test with, in order [p, q, p, q, ....]
-        primes = [64123, 50957, 39317, 33107]
+        # By starting with two of the same primes, we test that this is
+        # properly rejected.
+        primes = [64123, 64123, 64123, 50957, 39317, 33107]
 
         def getprime(_):
             return primes.pop(0)