[gdb/testsuite] Fix gdb.base/run-attach-while-running.exp

With a testsuite setup modified to make expect wait a little bit longer for
gdb output (see PR27957), I reliably run into:
...
27        return SYSCALL_CANCEL (nanosleep, requested_time, remaining);^M
(gdb) ^M
Thread 2 "run-attach-whil" stopped.^M
0x00007f13c85a74c0 in __GI___nanosleep () at nanosleep.c:27^M
27        return SYSCALL_CANCEL (nanosleep, requested_time, remaining);^M
FAIL: gdb.base/run-attach-while-running.exp: threaded=1: \
  run-or-attach=attach: non-stop=on: test: attach to process (timeout)
...

The problem is that we're trying to match the gdb_prompt using gdb_test which
uses '$gdb_prompt $'.  The terminating '$' prevents the match.

Fix this by rewriting the gdb_test into a gdb_test_multiple and dropping the
'$'.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-07  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27908
	* gdb.base/run-attach-while-running.exp (test): Don't match prompt
	after attach using '$gdb_prompt $'.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2021-06-07  Tom de Vries  <tdevries@suse.de>
+
+	PR testsuite/27908
+	* gdb.base/run-attach-while-running.exp (test): Don't match prompt
+	after attach using '$gdb_prompt $'.
+
 2021-06-07  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.base/info-types.exp.tcl: Remove "File .*" matching from regexp

gdb/testsuite/gdb.base/run-attach-while-running.exp

@@ -33,6 +33,8 @@ unset binfile
 #   - run-or-attach: "run" or "attach"
 
 proc_with_prefix test { non-stop threaded run-or-attach } {
+    global gdb_prompt
+
     if { ${run-or-attach} == "attach" && ![can_spawn_for_attach] } {
 	unsupported "attach not supported"
 	return
@@ -91,7 +93,11 @@ proc_with_prefix test { non-stop threaded run-or-attach } {
 	set test_spawn_id [spawn_wait_for_attach $::binfile]
 	set test_pid [spawn_id_get_pid $test_spawn_id]
 
-	gdb_test "attach $test_pid" "Attaching to program: .*" "attach to process"
+	gdb_test_multiple "attach $test_pid" "attach to process" {
+	    -re "Attaching to program: .*$gdb_prompt " {
+		pass $gdb_test_name
+	    }
+	}
 
 	gdb_exit
 	kill_wait_spawned_process $test_spawn_id