Do not shorten output comments that are in Godoc examples

[StevenACoffman]

An example of a example_test.go file that golines --shorten-comments --base-formatter=gofumpt -w . currently breaks is: https://go.dev/play/p/zjwVoZGcobT

As with typical tests, examples are functions that reside in a package’s _test.go files. Unlike normal test functions, though, example functions take no arguments and begin with the word Example instead of Test. If a comment begins with // Output: then it and any commented lines immediately following it should not be shortened by --shorten-comments .

At least, I think there's no valid way to shorten output lines!