Bug 16173: db_structure.t shouldn't have a fixed number of tests

 Bug 16173: db_structure.t shouldn't have a fixed number of tests

This patch makes db_structure.t more flexible regarding how many tests are run.
This is important because it would fail on each db structure change, masking potential
real problems.

To test:
- Run
  $ prove t/db_dependent/db_structure.t
=> FAIL: the number of tests ran doesn't match what is expected
- Apply the patch
- Run
  $ prove t/db_dependent/db_structure.t
=> SUCCESS: Tests count is not an issue [1]
- Sign off

[1] Right now the tests run successfuly, they might fail if a real bug is introduced
on the DB structure, in the meantime.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>