Allow running some DND tests interactively

Allow running some DND tests interactively

* src/xselect.c (x_get_local_selection): Respect new variable.
(syms_of_xselect): New variable
`x-treat-local-requests-remotely'.

* test/lisp/dnd-tests.el (x-begin-drag, gui-set-selection):
Don't redefine these functions under X.
(dnd-tests-verify-selection-data): Use
`x-get-selection-internal' under X.
(dnd-tests-extract-selection-data): New function.
(dnd-tests-begin-text-drag): Update accordingly.
(dnd-tests-begin-file-drag, dnd-tests-begin-drag-files):
Temporarily skip these tests under X.