tests/tcg/xtensa: enable mmu tests for MMUv3

MMU test suite is disabled for cores that have spanning TLB way, i.e.
for all MMUv3 cores. Instead of disabling it make testing region virtual
addresses explicit and invalidate TLB mappings for entries that conflict
with the test.

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