ert.el: Use `handler-bind` to record backtraces

ert.el: Use `handler-bind` to record backtraces

* lisp/emacs-lisp/ert.el (ert--should-signal-hook): Delete function.
(ert--expand-should-1): Don't bind `signal-hook-function`.
(ert--test-execution-info): Remove `next-debugger` slot.
(ert--run-test-debugger): Adjust to new calling convention.
Pass the `:backtrace-base` info to the debugger.
(ert--run-test-internal): Use `handler-bind` rather than let-binding
`debugger` and `debug-on-error`.

* lisp/emacs-lisp/ert-x.el (ert-remote-temporary-file-directory): Don't
use `defconst` if it's not meant to stay constant (e.g. we let-bind it
in tramp-tests.el).