Don't require bytecomp for running ert tests

Don't require bytecomp for running ert tests

"Fix ert-tests when running compiled" 2016-12-06 accidentally
introduced a dependency on `bytecomp' into `ert'.  As mentioned in
"Avoid ert test failures" 2017-04-18, the accidental dependency of ert
on bytecomp was masked by loading other libraries until recently.

* lisp/emacs-lisp/ert.el (ert--expand-should-1): Only use
`byte-compile-macro-environment' if it's bound.
* test/src/eval-tests.el: Add defvar for dynamic variable
`byte-compile-debug'.