testsuite, gdb.btrace: skip multi-inferior.exp on gdbserver targets

The gdb.btrace/multi-inferior.exp test creates multiple inferiors to check
that recording is per-inferior.

When run with the native-gdbserver board, this test hangs when trying to
add the second inferior over the remote connection.  Skip the test.

Note that the test runs fine with the native-extended-gdbserver board file
and we want to keep testing that configuration.

gdb/testsuite/ChangeLog:
2020-12-11  Markus Metzger  <markus.t.metzger@intel.com>

	* gdb.btrace/multi-inferior.exp: Skip if use_gdb_stub.