tests: Initialise breakpoint list for all test types

The list of client breakpoints was used for both client tests and plugin
tests - anything that uses the weston-test module - but was only
initialised in the client-test path. Make it unconditional.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: efde2fa0 ("tests: Add client<->compositor breakpoint support")