Silence recent comp-tests.el lexvar warnings

Silence recent comp-tests.el lexvar warnings

* test/lisp/emacs-lisp/comp-tests.el: Mark used native-compile
variables as special to pacify unknown lexvar warnings in the
default build.
(with-test-native-compile-prune-cache): Instrument macro arguments
for debugging and indent conventionally.  Reindent all callers.
(test-native-compile-prune-cache/dont-delete-in-parent-of-cache):
Simplify file name expansion.