[gdb/testsuite] Add missing skip_gdbserver_tests in gdb.multi/attach-no-multi-process.exp

I build gdb without gdbserver, and ran into:
...
(gdb) PASS: gdb.multi/attach-no-multi-process.exp: target_non_stop=off: \
  switch to inferior 2
spawn of  --once --multi localhost:2346 failed
ERROR: tcl error sourcing attach-no-multi-process.exp.
ERROR: tcl error code NONE
ERROR: Timeout waiting for gdbserver response.
...

Add the missing skip_gdbserver_tests.

Tested on x86_64-linux.

gdb/testsuite/gdb.multi/attach-no-multi-process.exp

@@ -22,6 +22,10 @@ load_lib gdbserver-support.exp
 
 standard_testfile
 
+if { [skip_gdbserver_tests] } {
+    return 0
+}
+
 if {![can_spawn_for_attach]} {
     return
 }