linters

Extend the testing framework to support a new routine ExpectOutputText which checks that the program output contains the specified text.

Use that to write new tests which verify that the token acquisition messages for all supported issuetrackers are shown.

Example token acquisition message for github:

Please go to https://github.com/settings/tokens to create a read-only access token 

followup from #149

	// makeExample replaces all ${linter} placeholders to a bound linter name.

The code below does not use ${linter} anywhere.
I could remove this, but would also like to avoid doing several unrelated things on the same PR, so keeping it as an issue to avoid forgetting about it.