[gdb/testsuite] Skip multi-target.exp without gdbserver

[gdb/testsuite] Skip multi-target.exp without gdbserver

Pre-commit 919adfe840 "Move gdbserver to top level", if we build gdb with
--disable-gdbserver, and run test-case gdb.multi/multi-target.exp, we run
into:
...
(gdb) PASS: gdb.multi/multi-target.exp: continue: non-stop=off: set remote-exec file in inferior 2
spawn of  --once --multi localhost:2346 failed
ERROR: tcl error sourcing /data/gdb_versions/devel/src/gdb/testsuite/gdb.multi/multi-target.exp.
ERROR: Timeout waiting for gdbserver response.
...

Fix this by using skip_gdbserver_tests in multi-target.exp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-10  Tom de Vries  <tdevries@suse.de>

	* gdb.multi/multi-target.exp: Skip if skip_gdbserver_tests.