Test warning suppressions with lexical binding

Test warning suppressions with lexical binding

* test/lisp/emacs-lisp/bytecomp-tests.el
(test-byte-comp-compile-and-load): Add lexical cookie to file being
compiled as part of the test.
(bytecomp-test--with-suppressed-warnings): Comment out the test
for suppressing warnings when attempting to let-bind `nil`,
as that (1) doesn't work and (2) is a silly thing to do anyway.