Improve TAP tests of pg_basebackup
This addresses some minor issues with the TAP tests of pg_basebackup:
- Remove three duplicated tests used for incorrect option combinations.
- Add more pattern checks for commands doomed to fail, to make sure that
the error generated is the expected one. These are for tests related to
the tablespace mapping and incorrect option combinations.
- Fix the description of one test for the case of backup target versus
format.
Issues noticed while reviewing this area of the tests.
Discussion: https://postgr.es/m/87bjvy50cs.fsf@wibble.ilmari.org