test(CI): check that the deferred linter runs in CI

[Vierkantor]

The deferred linter is used for Verso docstrings that contain forward references. We add a few correct and incorrect forward references to make sure the linter picks them up.

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[github-actions]

PR summary 921a172732

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.LintTest (new file)	61

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Declarations diff (regex)

+ foo

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean -- pending)

Computed after the build finishes.

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No changes to strong technical debt.
No changes to weak technical debt.

Current commit 921a172732
Reference commit 7ab51764b8

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.py pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).