[lldb] Stabilize threaded windows lldb-server tests

The tests enabled in 9e699595 are not passing reliably -- sometimes they
report seeing fewer threads than expected.

Based on my (limited) research, this is not a lldb bug, but simply a
consequence of the operating system reporting their presence
asynchronously -- they're reported when they are scheduled to run (or
something similar), and not at the time of the CreateThread call.

To fix this, I add some additional synchronization to the test inferior,
which makes sure that the created thread is alive before continuing to
do other things.