Pull out geometry column count query into test helper method

  * Create `geometry_column_count_query` in `test/ddl_test.rb` to clean
    up the multiple calls of

  `"SELECT COUNT(*) FROM geometry_columns WHERE f_table_name='spatial_test'"`
Merge pull request #122 from srt32/st-refactor-column-count-in-spec

Pull out geometry column count query into test helper method