Skip testing of watchpoint hit-count/ignore-count on multithreaded

Skip all watchpoint hit-count/ignore-count tests for multithreaded
API tests for now on arm64 Darwin.

On AArch64, insns that trigger a WP are rolled back and we are
notified.  lldb needs to disable the WP, insn step, re-enable it,
then report it to the user.  lldb only does this full step action
for the "selected thread", and so when a program stops with
multiple threads hitting a stop reason, some of them watchpoints,
any non-selected-thread will not be completed in this way.  But
all threads with the initial watchpoint exception will have their
hit-count/ignore-counts updated.  When we resume execution, the
other threads sitting at the instruction will again execute &
trigger the WP exceptoin again, repeating until we've gone through
all of the threads.

This bug is being tracked in llvm.org/pr49433 and inside apple
in rdar://93863107