Improve deadcode plugin for Minitest

[Morriar]

• Make plugin ignore classes that inherit from Minitest::Test instead of those ending with Test
• Modified the on_define_method method to ignore methods starting with test_ or included in the MINITEST_METHODS set only when inside a Minitest::Test subclass.
• Define on_send to mark methods used in assert_predicate and `refute_predicate`` as alive.