[gdb/testsuite] Fix gdb.threads/threadapply.exp with editing off

[gdb/testsuite] Fix gdb.threads/threadapply.exp with editing off

With test-case gdb.threads/threadapply.exp and editing set to on, we have:
...
(gdb) define remove^M
Type commands for definition of "remove".^M
End with a line saying just "end".^M
>remove-inferiors 3^M
>end^M
(gdb)
...
but with editing set to off, we run into:
...
(gdb) define remove^M
Type commands for definition of "remove".^M
End with a line saying just "end".^M
>remove-inferiors 3^M
end^M
>(gdb) FAIL: gdb.threads/threadapply.exp: thread_set=all: try remove: \
  define remove (timeout)
...

The commands are issued by this test:
...
	gdb_define_cmd "remove" {
	    "remove-inferiors 3"
	}
...
which does:
- gdb_test_multiple "define remove", followed by
- gdb_test_multiple "remove-inferiors 3\nend".

Proc gdb_test_multiple has special handling for multi-line commands, which
splits it up into subcommands, and for each subcommand issues it and then
waits for the resulting prompt (the secondary prompt ">" for all but the last
subcommand).

However, that doesn't work as expected in this case because the initial
gdb_test_multiple "define remove" fails to match all resulting output, and
consequently the secondary prompt resulting from "define remove" is counted as
if it was the one resulting from "remove-inferiors 3".

Fix this by matching the entire output of "define remove", including the
secondary prompt.

Tested on x86_64-linux.

PR testsuite/30288
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30288