Fixed #23357 -- Added small int introspection support to MySQL backend.

In the MySQL backend, updated the can_introspect_small_integer feature
flag to True. In data_types_reverse, map FIELD_TYPE.SHORT to a
SmallIntegerField. Added test to verify introspecting SmallIntegerFields
and fixed existing tests influenced by this change.

tests/introspection/models.py

@@ -11,6 +11,7 @@ class Reporter(models.Model):
     email = models.EmailField()
     facebook_user_id = models.BigIntegerField(null=True)
     raw_data = models.BinaryField(null=True)
+    small_int = models.SmallIntegerField()
 
     class Meta:
         unique_together = ('first_name', 'last_name')