gdb.base/async.exp: Fix races.

gdb.base/async.exp: Fix races.

This test is currently racy:

 PASS: gdb.base/async.exp: step&
 stepi&
 (gdb) 0x0000000000400547        14       x = 5; x = 5;
 completed.
 PASS: gdb.base/async.exp: stepi&
 nexti&
 (gdb) 15         y = 3;
 completed.FAIL: gdb.base/async.exp: nexti&

The problem is here:

 	-re "^$command\r\n${before_prompt}${gdb_prompt}${after_prompt}completed\.\r\n" {
 	    pass "$command"
 	}
	-re "$gdb_prompt.*completed\.$" {
 	    fail "$command"
	}

Note how the fail pattern is a subset of the pass pattern.  If the
expect buffer happens to end up with:

  "^$command\r\n${before_prompt}${gdb_prompt}${after_prompt}completed\."

that is, the final "\r\n" has't reached the expect buffer yet, but
"completed." has, then the fail pattern matches...

gdb/testsuite/
2014-03-19  Pedro Alves  <palves@redhat.com>

	* gdb.base/async.exp (test_background): Expect \r\n after
	"completed." in the fail pattern.