Make TestThreadSpecificBreakpoint.py more focused.

This test would fail before if conditional breakpoints weren't
working correctly, and the nature of the test (spinning up 10
threads, etc) opens the door to raciness.

This patch vastly simplifies the test, removes the need for relying
on conditional expression evaluation, and as a result makes the
correctness of the test vastly easier to reason about and reduces
flakiness.

Differential Revision: http://reviews.llvm.org/D15241
Reviewed By: Jim Ingham

llvm-svn: 254792