Improve end-to-end tests

[grobie]

The end-to-end tests currently only work under Linux, but we should also test all other collectors for other OSes. Instead of comparing the output to one single golden template, each collector probably needs their own file.

Rewriting the end-to-end tests to Go will probably make things easier.

[mdlayher]

I'd be happy to help out with that project.

Something I've adopted on my own (that may or may not be ideal), is a test function that grabs the byte slice output from a collector using a net/http/httptest server: https://github.com/mdlayher/rtorrent_exporter/blob/master/rtorrentexporter_test.go#L12.

From there, you can explicitly search for sub-slices using bytes or regexp as needed.

At the very least, it does give you the ability to t.Skip where needed.

[discordianfish]

@mdlayher Is this still something you want to look into?