Have the test suite find an empty Redis database.

Since the test suite `flushdb()`'s after running each test, we should
make sure the database is empty before we even start running tests.
This patch will make sure to never destroy any local production data
inside the running Redis instance.

This fixes #25.

tests/__init__.py

@@ -14,6 +14,18 @@ def failing_job(x):
     return x / 0
 
 
+def find_empty_redis_database():
+    """Tries to connect to a random Redis database (starting from 4), and
+    will use/connect it when no keys are in there.
+    """
+    for dbnum in range(4, 17):
+        testconn = Redis(db=dbnum)
+        empty = len(testconn.keys('*')) == 0
+        if empty:
+            return testconn
+    assert False, 'No empty Redis database found to run tests in.'
+
+
 class RQTestCase(unittest.TestCase):
     """Base class to inherit test cases from for RQ.
 
@@ -27,7 +39,7 @@ class RQTestCase(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         # Set up connection to Redis
-        testconn = Redis()
+        testconn = find_empty_redis_database()
         conn.push(testconn)
 
         # Store the connection (for sanity checking)