Fix internal warning when "gdb -p xxx"

The issue that was fixed by b4ab256ded5020a82ff7ce8dc485e7882fc5b6a7
can not be found in regression test.
Update attach.exp to test it.

https://sourceware.org/ml/gdb-patches/2014-03/msg00438.html

2014-03-21  Hui Zhu  <hui@codesourcery.com>

	* gdb.base/attach.exp (do_command_attach_tests): New.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2014-03-21  Hui Zhu  <hui@codesourcery.com>
+
+	* gdb.base/attach.exp (do_command_attach_tests): New.
+
 2014-03-20  Tom Tromey  <tromey@redhat.com>
 	    Pedro Alves  <palves@redhat.com>
 

gdb/testsuite/gdb.base/attach.exp

@@ -384,6 +384,51 @@ proc do_call_attach_tests {} {
     remote_exec build "kill -9 ${testpid}"
 }
 
+proc do_command_attach_tests {} {
+    global gdb_prompt
+    global binfile
+    global verbose
+    global GDB
+    global INTERNAL_GDBFLAGS
+    global GDBFLAGS
+
+    if ![isnative] then {
+	unsupported "command attach test"
+	return 0
+    }
+
+    # Start the program running and then wait for a bit, to be sure
+    # that it can be attached to.
+
+    set testpid [eval exec $binfile &]
+    exec sleep 2
+    if { [istarget "*-*-cygwin*"] } {
+	# testpid is the Cygwin PID, GDB uses the Windows PID, which might be
+	# different due to the way fork/exec works.
+	set testpid [ exec ps -e | gawk "{ if (\$1 == $testpid) print \$4; }" ]
+    }
+
+    gdb_exit
+    if $verbose>1 then {
+        send_user "Spawning $GDB $INTERNAL_GDBFLAGS $GDBFLAGS --pid=$testpid\n"
+    }
+
+    eval "spawn $GDB $INTERNAL_GDBFLAGS $GDBFLAGS --pid=$testpid"
+    set test "starting with --pid"
+    expect {
+	-re "Reading symbols from.*$gdb_prompt $" {
+	    pass "$test"
+	}
+	timeout {
+	    fail "$test (timeout)"
+	}
+    }
+
+    # Get rid of the process
+
+    remote_exec build "kill -9 ${testpid}"
+}
+
 
 # Start with a fresh gdb
 
@@ -404,4 +449,8 @@ gdb_start
 gdb_reinitialize_dir $srcdir/$subdir
 do_call_attach_tests
 
+# Test "gdb --pid"
+
+do_command_attach_tests
+
 return 0