[gdb/testsuite] Fix -prompt handling in gdb_test

With check-read1 I run into:
...
   [infrun] maybe_set_commit_resumed_all_targets: not requesting
commit-resumed for target native, no resumed threads^M
(gdb) FAIL: gdb.base/ui-redirect.exp: debugging: continue
[infrun] fetch_inferior_event: exit^M
...

The problem is that proc gdb_test doesn't pass down the -prompt option to proc
gdb_test_multiple, due to a typo making this lappend without effect:
...
    set opts {}
    lappend "-prompt $prompt"
...

Fix this by actually appending to opts.

Tested on x86_64-linux.

gdb/testsuite/lib/gdb.exp

@@ -1393,7 +1393,7 @@ proc gdb_test { args } {
     set user_code [join $user_code]
 
     set opts {}
-    lappend "-prompt $prompt"
+    lappend opts "-prompt" "$prompt"
     if {$lbl} {
 	lappend opts "-lbl"
     }