Fix TestBatchMode on linux

New test introduced in r257120 was failing on linux. The reason for that the regex for setting
the breakpoint was being applied to the "default file", which in this case was the asm file
containing the definition of the sleep() syscall (because after attach, we are stopped in the
sleep function). I have changed this use the more customary way of setting the breakpoint and
specifying the source file name explicitly.

llvm-svn: 257160