Bug 19985: Make TestBuilder.t pass even if default_circ_rules is not empty

Bug 19985: Make TestBuilder.t pass even if default_circ_rules is not empty

This table cannot contain more than 1 row, so we need to remove its data
before trying to generate a new entry.

Test plan:
Set a default circ rule
 prove t/db_dependent/TestBuilder.t
should return green

Without this patch you get:
    #   Failed test 'TestBuilder should be able to create an object for every source'
    #   at t/db_dependent/TestBuilder.t line 78.
    #          got: '1'
    #     expected: '0'
    # The following sources have not been generated correctly: DefaultCircRule
    # Looks like you failed 1 test of 1.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>