Speed up TestWatchpointMultipleThreads

Summary:
The inferior was sleeping before doing any interesting work. I remove that
to make the test faster.

While looking at the purpose of the test (to check that watchpoints are
propagated to all existing threads - r140757) I noticed that the test has
diverged from the original intention and now it creates the threads *after* the
watchpoint is set (this probably happened during the std::thread refactor).
After some discussion, we decided both scenarios make sense, so I modify the
test to test both.

The watchpoint propagation functionality is not really debug info depenent, so
I also stop replication of this test. This brings the test's time from ~108s
down to 4s.

Reviewers: davide, jingham

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D43857

llvm-svn: 326514