Testcase for detaching while stepping over breakpoint

This adds a testcase that exercises detaching while GDB is stepping
over a breakpoint, in all combinations of:

  - maint target non-stop off/on
  - set non-stop on/off
  - displaced stepping on/off

This exercises the bugs fixed in the previous 8 patches.

gdb/testsuite/ChangeLog:

	* gdb.threads/detach-step-over.c: New file.
	* gdb.threads/detach-step-over.exp: New file.
2 files changed