Add mi-threads-interrupt.exp test (PR 20039)

Add mi-threads-interrupt.exp test (PR 20039)

Add a new test for PR 20039.  The test spawns new threads, then tries to
interrupt, continue, and interrupt again.  This use case was fixed by
commit 5fe966540d6b748f825774868463003700f0c878 in master, but gdb 7.11
is affected (so if you try it on the gdb-7.11-branch right now, the test
will fail).

New in v2, the test now handles mi-async on mode properly.  The failure
was specific to mi-async off, but I don't think it's bad to test the
same thing under async on mode.  I added a little hack when running in
async mode to work around bug 20045.

I also removed one continue/interrupt pair, as a single one was enough to
trigger the problem.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-threads-interrupt.c: New file.
	* gdb.mi/mi-threads-interrupt.exp: New file.