Split gdb.multi/multi-target.exp into separate testcases

Split gdb.multi/multi-target.exp into separate testcases

gdb.multi/multi-target.exp sets up a debug environment with multiple
gdbservers, multiple native processes, and multiple cores, which has
proved useful for exercising a number of multi-target scenarios.

But, as we add more tests to gdb.base/multi-target.exp, it is growing
a bit too large (making a bit cumbersome to debug) and too slow to run
(if you have glibc debug info).

This commit thus splits the multi-target.exp into several testcases,
one per use case.  The common setup code is moved to a new
multi-target.exp.tcl file that is included by all the resulting
multi-target testcases.

gdb/testsuite/ChangeLog:

	* gdb.multi/multi-target-continue.exp: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target-info-inferiors.exp: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target-interrupt.exp: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target-no-resumed.exp: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target-ping-pong-next.exp: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target.exp.tcl: New file, factored out from
	multi-target.exp.
	* gdb.multi/multi-target.exp: Delete.