Improve message for unit test failures for lucene.testsettings.json, #999

[paulirwin]

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Improves the failed unit test message to provide an example lucene.testsettings.json.

Fixes #999

Description

The lucene.testsettings.json seems to work more reliably and is a little more user-friendly than the runsettings file, so this improves the failed unit test message to provide a generated lucene.testsettings.json file. This also uses C# 11 multi-line strings instead of manual string concatenation.