Added tests to show race conditions

[pstibrany]

Added two tests to show race conditions in Tell and Size methods. (#3)

Looking at Go API docs, I don't see any guarantees about os.File being safe for concurrent access from different goroutines, so unless we want to have mutex around all file usages, safest way may be to remove (!!) these two methods completely.

[pstibrany]

(Oh yes, while at it, I've converted this to use go modules)

[slim-bean]

We need the Tell method to update the positions file in promtail, and the Size method sort of helps with an alert we have for making sure we are tailing files.

Adding a mutex around only the places where we set the file addresses the race from the test perspective. Although to your point we could be calling a method on that as it's changed (didn't mutex the other methods) Though I think this is probably ok? Maybe we get the size from a different file but the way we use this I think that will end up being not important (we poll those functions every 10 seconds or so)

We could also mutex the other places to be thorough, I opened #5 which gets the tests to pass. WDYT @pstibrany ?

[pstibrany]

@slim-bean Should we merge this as well (if it passes now)?

[pstibrany]

(ok, this also does go mod conversion, so maybe not. perhaps needs a closer look...)

[slim-bean]

I'm find with the go mod conversion, we can merge this too!