Add more tests for backtrace-mode

Add more tests for backtrace-mode

* test/lisp/emacs-lisp/backtrace-tests.el (backtrace-tests--variables)
(backtrace-tests--backward-frame, backtrace-tests--forward-frame)
(backtrace-tests--pretty-print-and-collapse)
(backtrace-tests--verify-pp-and-collapse)
(backtrace-tests--print-circle, backtrace-tests--make-regexp)
(backtrace-tests--expand-ellipsis): New tests.
(backtrace-tests--to-string): Use backtrace-tests--make-backtrace.
(backtrace-tests--get-substring): New function.

Change the method of generating sample backtraces in backtrace tests
to work whether or not the tests are byte-compiled.
* test/lisp/emacs-lisp/backtrace-tests.el (backtrace-tests--func1)
(backtrace-tests--func2, backtrace-tests--func3)
(backtrace-tests--create-backtrace-frames): Remove.
(backtrace-tests--uncompiled-functions): New constant.
(backtrace-tests--make-backtrace, backtrace-tests--setup-buffer):
New functions.
(backtrace-tests--backtrace-lines)
(backtrace-tests--backtrace-lines-with-locals): New functions.
(backtrace-tests--line-count): New constant.
(backtrace-tests--result, backtrace-tests--result-with-locals):
New functions.
(backtrace-tests--header): New constant.
(backtrace-tests--insert-header): Use backtrace-tests--header.
(backtrace-tests--with-buffer): Remove.