gdb.base/async.exp: Make test messages unique.

gdb.base/async.exp: Make test messages unique.

 $ cat gdb.sum| grep PASS| sort | uniq -c |sort -n
       1 PASS: gdb.base/async.exp: finish&
       1 PASS: gdb.base/async.exp: jump&
       1 PASS: gdb.base/async.exp: next&
       1 PASS: gdb.base/async.exp: nexti&
       1 PASS: gdb.base/async.exp: set exec-done-display off
       1 PASS: gdb.base/async.exp: set exec-done-display on
       1 PASS: gdb.base/async.exp: set target-async on
       1 PASS: gdb.base/async.exp: stepi&
       1 PASS: gdb.base/async.exp: until&
       2 PASS: gdb.base/async.exp: step&

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

	* gdb.base/async.exp (step& tests): Pass explicit test messages.