gdb/testsuite: add test for run/attach while program is running

A WIP patch series broke the use case of doing "run" or "attach" while
the program is running, but it wasn't caught by the testsuite, which
means it's not covered.  Add a test for that.

gdb/testsuite/ChangeLog:

	* gdb.base/run-attach-while-running.exp: New.
	* gdb.base/run-attach-while-running.c: New.

Change-Id: I77f098ec0b28dc2d4575ea80e941f6a75273e431