[gdb/testsuite] Fix gdb.threads/leader-exit-attach.exp with check-read1

With test-case gdb.threads/leader-exit-attach.exp and check-read1, I run into:
...
(gdb) attach 18591^M
Attaching to program: leader-exit-attach, process 18591^M
warning: process 18591 is a zombie - the process has already terminatedKFAIL: $exp: attach (PRMS: gdb/31555)
^M
ptrace: Operation not permitted.^M
(gdb) FAIL: $exp: get valueof "$_inferior_thread_count"
...

The problem is that the gdb_test_multiple in the test-case doesn't consume the
prompt in all clauses:
...
gdb_test_multiple "attach $testpid" "attach" {
    -re "Attaching to process $testpid failed.*" {
	# GNU/Linux gdbserver.  Linux ptrace does not let you attach
	# to zombie threads.
	setup_kfail "gdb/31555" *-*-linux*
	fail $gdb_test_name
    }
    -re "warning: process $testpid is a zombie - the process has already terminated.*" {
	# Native GNU/Linux.  Linux ptrace does not let you attach to
	# zombie threads.
	setup_kfail "gdb/31555" *-*-linux*
	fail $gdb_test_name
    }
    -re "Attaching to program: $escapedbinfile, process $testpid.*$gdb_prompt $" {
	pass $gdb_test_name
	set attached 1
    }
}
...

Fix this by using -wrap in the first two clauses.

While we're at it, also use -wrap in the third clause.

Tested on x86_64-linux.