Fix `warning: ambiguous first argument`

```
test/cases/adapters/postgresql/case_insensitive_test.rb:12: warning: ambiguous first argument; put parentheses or a space even after `/' operator
test/cases/adapters/postgresql/case_insensitive_test.rb:16: warning: ambiguous first argument; put parentheses or a space even after `/' operator
test/cases/adapters/postgresql/case_insensitive_test.rb:20: warning: ambiguous first argument; put parentheses or a space even after `/' operator
test/cases/adapters/postgresql/case_insensitive_test.rb:24: warning: ambiguous first argument; put parentheses or a space even after `/' operator
```

activerecord/test/cases/adapters/postgresql/case_insensitive_test.rb

@@ -9,18 +9,18 @@ def test_case_insensitiveness
 
     column = Default.columns_hash["char1"]
     comparison = connection.case_insensitive_comparison table, :char1, column, nil
-    assert_match /lower/i, comparison.to_sql
+    assert_match(/lower/i, comparison.to_sql)
 
     column = Default.columns_hash["char2"]
     comparison = connection.case_insensitive_comparison table, :char2, column, nil
-    assert_match /lower/i, comparison.to_sql
+    assert_match(/lower/i, comparison.to_sql)
 
     column = Default.columns_hash["char3"]
     comparison = connection.case_insensitive_comparison table, :char3, column, nil
-    assert_match /lower/i, comparison.to_sql
+    assert_match(/lower/i, comparison.to_sql)
 
     column = Default.columns_hash["multiline_default"]
     comparison = connection.case_insensitive_comparison table, :multiline_default, column, nil
-    assert_match /lower/i, comparison.to_sql
+    assert_match(/lower/i, comparison.to_sql)
   end
 end