More result types

UNTESTED
This would be set for all BT's, with assert_num = NULL. This result will be deleted when BT execution is complete.

XFAIL, XPASS
Optionally (but not by default), expectations can generate XFAIL or XPASS instead of FAIL and PASS.

UNSUPPORTED
Within BT's, support filters on VERSION() string:
stk_unit.exec_if('%mariadb%');
stk_unit.exec_if('5.5.%');
If none of the filters passes, BT execution stops and an UNSUPPORTED result with assert_num=NULL is inserted. It replaces UNTESTED.