[lldb][AArch64] Only run MTE memory region test if we have MTE

This test is flakey because it tries to read the proc/smaps
file of the first lldb-server process it finds. This process
can finish before we finish doing that.

http://lab.llvm.org:8011/#/builders/96/builds/6634/steps/6/logs/stdio

For now limit this to MTE targets which basically means
QEMU via lldb-dotest, which doesn't have this issue.

I'll fix the race condition shortly.