Flymake: unbreak tests

Flymake: unbreak tests

Flymake's normal behaviour implies catching the errors of misbehaving
backends.  This behavior is tested by Flymake's automated tests, built
on top of ERT, which means that debug-on-error is always t in the
bodies of said tests (I don't know the rationale for this, but it's
been like this for some time)

Flymake used to shun usage of 'condition-case-unless-debug' because of
this.  But since that macro is pretty useful, I started using it
again, and as a consequence, tests started breaking.

The solution is to:

1. stop requiring the deprecated legacy backend 'flymake-proc' (which
by design, always errors, except in rare circumstances).

2. deliberately set debug-on-error to nil around the "dummy backends""
test.

* lisp/progmodes/flymake.el (flymake-proc): Don't require it by
default.

* test/lisp/progmodes/flymake-tests.el:
(dummy-backends): Make robust to ert's debug-on-error setting.