tests/tcg/xtensa: fix vectors and checks in timer test

tests/tcg/xtensa: fix vectors and checks in timer test

Timer test assumes that timer 0 IRQ has level 1 and other timers have
higher level IRQs. This assumption is not correct and the levels may be
arbitrary. Fix that assumption by providing TIMER*_VECTOR macro and
using it for vector selection and by making the check for the timer
exception cause conditional.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>