Fix gdb.trace/trace-buffer-size.exp race.

Just the usual missing $gdb_prompt match:

 (gdb) tstatus
 No trace has been run on the target.
 Collected 0 trace frames.
 Trace buffer has 5242880 bytes of 5242880 bytes free (0% full).
 Trace will stop if GDB disconnects.
 Not looking at any trace frame.
 PASS: gdb.trace/trace-buffer-size.exp: get default buffer size
 (gdb) set trace-buffer-size 4
 (gdb) FAIL: gdb.trace/trace-buffer-size.exp: set trace buffer size 1

This fixes it.

gdb/testsuite/
2013-03-21  Pedro Alves  <palves@redhat.com>

	* gdb.trace/trace-buffer-size.exp (get default buffer size):
	Expect $gdb_prompt in gdb_test_multiple.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2013-03-21  Pedro Alves  <palves@redhat.com>
+
+	* gdb.trace/trace-buffer-size.exp (get default buffer size):
+	Expect $gdb_prompt in gdb_test_multiple.
+
 2013-03-21  Doug Evans  <dje@google.com>
 
 	* gdb.base/maint.exp: Update tests for per-command stats.

gdb/testsuite/gdb.trace/trace-buffer-size.exp

@@ -39,7 +39,7 @@ set test "get default buffer size"
 
 # Save default trace buffer size in 'default_size'.
 gdb_test_multiple "tstatus" $test {
-    -re ".*Trace buffer has ($decimal) bytes of ($decimal) bytes free.*" {
+    -re ".*Trace buffer has ($decimal) bytes of ($decimal) bytes free.*$gdb_prompt $" {
         set default_size $expect_out(2,string)
         pass $test
     }