Keep indenting text when 'shr-fill-text' is nil (bug#69555)

Keep indenting text when 'shr-fill-text' is nil (bug#69555)

The 'shr-fill-...' functions handle both hard-filling (adding
newlines to break long lines) and indentation.  Setting
'shr-fill-text' to nil currently causes these functions to be
short-circuited completely, so e.g. blockquotes are no longer
indented, whereas the intent of this user option is only to
prevent hard-filling to let visual-line-mode reflow text.

* lisp/net/shr.el (shr-fill-lines): Document that the function
handles more than just filling; move the 'shr-fill-text' check...
(shr-fill-line): ... here, after indentation has been taken care
of.
* test/lisp/net/shr-resources/blockquote.html:
* test/lisp/net/shr-resources/blockquote.txt: New test resources.
* test/lisp/net/shr-tests.el (shr-test--rendering-check): Rename
from 'shr-test', to make the relationship with the 'rendering'
testcase clearer; prefer 'file-name-concat' to 'format'; raise
ERT failure if need be, calling (ert-fail ...) directly instead
of (should (not (list ...))).
(shr-test--rendering-extra-configs): New variable to easily check
that user customizations do not degrade rendering.
(rendering): Consult that new variable; delegate failure-raising
to reduce duplication.