Don't rewrite `set` to `setq` of lexical variables

Don't rewrite `set` to `setq` of lexical variables

Only perform the rewrite

   (set 'VAR X) -> (setq VAR X)

for dynamic variables, as `set` isn't supposed to affect
lexical vars (and never does so when interpreted).

* lisp/emacs-lisp/byte-opt.el (byte-optimize-set):
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--xx): New.
(bytecomp-tests--test-cases): Add test cases.
* test/lisp/emacs-lisp/bytecomp-resources/warn-variable-set-nonvariable.el:
Remove obsolete test.