Fix solib-display.exp remote check

This test currently uses [is_remote target] to check if the test is
supported.  This is not quite correct, as the limitation is actually
that it requires support for "running", ruling out stub-like targets.
Therefore, it should check for use_gdb_stub.

This has no visible effect right now, but it will once we make the
native-gdbserver board non-dejagnu-remote.

gdb/testsuite/ChangeLog:

	* gdb.base/solib-display.exp: Check for [use_gdb_stub] instead
	of [is_remote target],