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