Tests: improve require_mantis_core() function

New version no longer requires use of eval() to declare globals.

Also fixes a system warning caused by $g_queries_array not being
declared as global in database api.

Backport from master branch 0b53ba9fb21a77358591ad5c5a48e272e733db9f,
added workaround to avoid warnings caused by lack of require_api()
function.