failed on HANDLER + I_S

This assert was triggered when an I_S query tried to acquire a
metadata lock on a table which was already locked by a HANDLER
statement in the same connection.

First the HANDLER took a MDL_SHARED lock. Afterwards, the I_S query
requested a MDL_SHARED_HIGH_PRIO lock. The existing MDL_SHARED ticket
is found in find_ticket() since it satisfies 
ticket->has_stronger_or_equal_type(mdl_request->type) as MDL_SHARED
and MDL_SHARED_HIGH_PRIO have equal strengths, just different priority.

However, two asserts later check lock type strengths using relational
operators (>= and <=) rather than MDL_ticket::has_stronger_or_equal_type().
These asserts are triggered since MDL_SHARED >= MDL_SHARED_HIGH_PRIORITY
is false (mapped to 1 and 2 respectively).

This patch updates the asserts to use MDL_ticket::has_stronger_or_equal_type()
rather than relational operators to check lock type strength.

Test case added to include/handler.inc.