gdb-remote test noise suppression on MacOSX.

This change adds a member to the base test case for gdb-remote that
indicates whether a stub makes two X stop notification reports on kill
commands.  This is set to true for debugserver tests.

The test for killing an attached process after it's first stop notification
has been modified to look at that flag and add an extra X packet matcher
so the "unmatched packet warning" doesn't get emitted for the second X on
MacOSX with debugserver.

I also broke those tests out of the monolithic TestLldbGdbServer mega test
case and put it in its own, new TestGdbRemoteKill.py file and test case.

Tested:
Ubuntu 14.04 x86_64, clang-3.5 built lldb, no test failures.
MacOSX 10.9.4, Xcode 6.0 Beta 3 built lldb, no test failures.

llvm-svn: 213166