Use ambient lexical-binding value in ert-deftest body (bug#50738)

Use ambient lexical-binding value in ert-deftest body (bug#50738)

* lisp/emacs-lisp/ert.el (ert-deftest):
Evaluate the body of `ert-deftest` with the `lexical-binding` value of
the source file (or more precisely the value in force when the
definition is evaluated), which is what everyone expected, instead of
always using dynamic binding which is what they got until now.
* test/lisp/emacs-lisp/ert-tests.el
(ert-test-deftest-lexical-binding-t): New test.