Simplify test dependency generation

Simplify test dependency generation

Generate default dependencies by using GNU extensions to ‘make’
rather than via a hacky auxiliary program and script.
* .gitignore: Remove test/make-test-deps.mk.
* test/Makefile.in (ELFILES, LOGFILES, TESTS):
Use :=, not =, to avoid multiple redundant invocations of ‘find’.
(test_template): Infer dependency directly instead of via
make-test-deps.mk.
(check-doit): Prepend ‘@’ to avoid excessively long ‘make’ output.
(clean): No need to clean make-test-deps.mk.
(make-test-deps.mk): Remove rule.
* test/make-test-deps.emacs-lisp: Remove.