[TestWatchpointMultipleThreads] Use default test executable names.

Summary:
Android has limits on file name lengths. This commit reduces the names of
the test executables to satisfy these limits.

Test Plan: dotest.py -p TestWatchpointMultipleThreads

Reviewers: chaoren

Reviewed By: chaoren

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D9540

llvm-svn: 236783