util-mysql: remove mapistore dependecy

create_schema returns bool instead of mapistore_error.
Test adapted and moved to the proper test suite.

This change is needed to avoid dependecy of mapiproxy -> mapistore,
because util/mysql is used by both and mapistore already depends on
mapiproxy.
yeah, agree, I was about to propose changing them too, but thinking of it, 
it is is very hard to exloit the pattern and write something like

    MAPISTORE_RETVAL_IF(do_something(), do_otherthing(), NULL)

especially given the fact that pattern everywhere is

    MAPISTORE_RETVAL_IF(do_something(), MAPISTORE_ERROR_CODE, NULL)