Fix gdb.base/a2-run.exp race

Fix gdb.base/a2-run.exp race

This patch fixes this racy failure, with the native-extended-gdbserver
board:

 (gdb) run
 Starting program: build/gdb/testsuite/outputs/gdb.base/a2-run/a2-run
 Remote debugging from host 127.0.0.1
 Process build/gdb/testsuite/outputs/gdb.base/a2-run/a2-run created; pid = 23832
 Reading /lib64/ld-linux-x86-64.so.2 from remote target...
 warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
 Reading /lib64/ld-linux-x86-64.so.2 from remote target...
 Reading /lib64/libm.so.6 from remote target...
 Reading /lib64/libc.so.6 from remote target...
 [Inferior 1 (process 23832) exited with code 01]
 (gdb) FAIL: gdb.base/a2-run.exp: run "a2-run" with no args
 PASS: gdb.base/a2-run.exp: no spurious messages at program exit
 run 5
 Starting program: build/gdb/testsuite/outputs/gdb.base/a2-run/a2-run 5
 Reading /lib64/ld-linux-x86-64.so.2 from remote target...
 usage:  factorial <number>

 Child exited with status 1

Note that the output is correct; it's just that inferior output
appeared after gdb's output, and the test doesn't handle that
correctly.

This comment isn't really correct, unfortunately:

	# waiting.  If we had already seen the status wrapper exit,
	# gdb_test_multiple/expect has no spawn ids left, and thus
	# returns.

That's true of expect in general, but I had missed / forgot that
gdb_test_multiple internally has extra matches using "-i
$gdb_spawn_id", so even if the caller clears all the indirect spawn id
lists, gdb_test_multiple will continue waiting.

So do a conditional exp_continue manually instead.

gdb/testsuite/ChangeLog:
2015-10-09  Pedro Alves  <palves@redhat.com>

	* gdb.base/a2-run.exp (maybe_exp_continue): New procedure.
	(top level): Use it in the run with no args test.