Fix linter stdout/stderr assertions

[ascandella]

I noticed while working on a new linter that my test was passing even though the stdout/stderr was incorrect.

This seems to be fallout from this refactor:
73e9574

I had to update the timezone date regex to allow my timezone as well.

I also updated the gofmt linter, since it sends its output to stdout. I also fixed the tests for it.

I updated the prettier test to remove the fix assertion on stdout because, on my machine, it outputs something like:

    Expected: ""
    Received: ".prettierrc.json 26ms
    file1.js 18ms
    file2.css 45ms
    package.json 6ms"

to stdout.

[ascandella]

The linter check doesn't seem to be checking out my branch properly (since it's on my fork, not on this repo):

  stderr: "error: pathspec 'fix-assertions' did not match any file(s) known to git\n"

[samuelmeuli]

Very nice PR, thanks so much for these fixes @ascandella!

[samuelmeuli]

The "Lint" workflow is currently failing because of #13, that's not an issue with your code :)

[samuelmeuli]

Thanks!