Fixup #9614: make config file extraction from help text more robust (#9667)

* Fixup #9614: make config file extraction from help text more robust

The previous test (PR #9614) did not work when the config file was
absent, because then the help text would add one more line at the
end (see issue #535).

The new test looks for the exact line printed before the line with the
config file.

We test both scenarios, with config file present or absent.

* Trim lines before searching the marker to work around CRLF issues.

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