[lldb][AArch64] Fix flakiness in TestSVEThreadedDynamic

This test runs to a breakpoint on thread 0. Thread 0 then starts
thread 2 and 3, which both have breakpoints in them.

In https://lab.llvm.org/buildbot/#/builders/96/builds/41674
I think that we managed to do the first check on thread 2 before
thread 3 had started. Therefore "thread continue 3" failed.

So wait for all three to startup before we check their status.

I considered putting a timeout on the while like the wait_for... methods,
but the test itself already has a global timeout. Plus, I'd rather
not be tuning a timeout per piece of hardware this runs on.

99% of the time we will already have 3 threads when the check is done.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154705