[1.5.x] Avoided having an indexed TextField installed unless using postgres

An index on TextField results in a warning message when running tests
on MySQL or SQLite, and the test using the TextField was PostgreSQL
only in any case.

Backpatch of 13a2b11425

tests/regressiontests/indexes/models.py

@@ -1,3 +1,4 @@
+from django.db import connection
 from django.db import models
 
 
@@ -11,7 +12,9 @@ class Article(models.Model):
         ]
 
 
-class IndexedArticle(models.Model):
-    headline = models.CharField(max_length=100, db_index=True)
-    body = models.TextField(db_index=True)
-    slug = models.CharField(max_length=40, unique=True, db_index=True)
+# Indexing a TextField on Oracle or MySQL results in index creation error.
+if connection.vendor == 'postgresql':
+    class IndexedArticle(models.Model):
+        headline = models.CharField(max_length=100, db_index=True)
+        body = models.TextField(db_index=True)
+        slug = models.CharField(max_length=40, unique=True, db_index=True)