Fix TestThreadSpecificBreakpoint.py on Linux after rL252355.

Summary:
On Linux, if a thread-specific conditional breakpoint was hit, it
won't necessarily be the thread that hit the breakpoint itself that
evaluates the conditional expression, so the thread that hit the
breakpoint could still be asked to stop, even though it hasn't been
allowed to run since the previous stop.

Reviewers: sivachandra, jingham

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14472

llvm-svn: 252391