[lldb] Speculative fix to TestGuiExpandThreadsTree

This test relies on being able to unwind from an arbitrary place inside
libc. While I am not sure this is the cause of the observed flakyness,
it is known that we are not able to unwind correctly from some places in
(linux) libc.

This patch adds additional synchronization to ensure that the inferior
is in the main function (instead of pthread guts) when lldb tries to
unwind it. At the very least, it should make the test runs more
predictable/repeatable.