Improve error message for 'new-test' command

[erikd]

Suggest adding '--enable-tests'. [ci skip]

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• Patches conform to the coding conventions.
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[dcoutts]

Fine, or perhaps better to recommend the equivalent in the cabal.project[.local].

[erikd]

Hmm, yeah. For projects with a single cabal this is the right message, but maybe it should be changed for multi cabal file projects.

[erikd]

Had private IM discussion with @dcoutts about this issue and this is the change we came to agree on.