SQL: fix meta/has_table
Further work for cleaning up the sql code (#6292).
- the `meta` attribute is not updated automatically, with the consequence that eg when you delete a table from sql directly, the `has_table` function does not work anymore:
  - I added a test for that
  - I converted the `meta` attribute to one which is always updated when called. @mangecoeur looks OK?
- now the tests are skipped when no connection could be made. @jreback, I just did a `raise nose.SkipTest` inside the setup function of the test class. -> moved and merged this in #6651 
