Fixed contrib.gis tests breaking in some edge cases.

The tests would error out with an ImproperlyConfigured error
if the user had psycopg installed but not geos.
Thanks to mcgohier for the report.

django/contrib/gis/tests/tests.py

@@ -1,13 +1,25 @@
+import sys
 import unittest
 
 from django.core.exceptions import ImproperlyConfigured
 from django.db import ProgrammingError
+from django.utils import six
 
 try:
     from django.contrib.gis.db.backends.postgis.operations import PostGISOperations
     HAS_POSTGRES = True
 except ImportError:
     HAS_POSTGRES = False
+except ImproperlyConfigured as e:
+    # If psycopg is installed but not geos, the import path hits
+    # django.contrib.gis.geometry.backend which will "helpfully" convert
+    # an ImportError into an ImproperlyConfigured.
+    # Here, we make sure we're only catching this specific case and not another
+    # ImproperlyConfigured one.
+    if e.args and e.args[0].startswith('Could not import user-defined GEOMETRY_BACKEND'):
+        HAS_POSTGRES = False
+    else:
+        six.reraise(*sys.exc_info())
 
 
 if HAS_POSTGRES: