Unit tests for Promtail (#244)

* Adding a test for target.go
Fixing some shutdown issues where tailers were stopped but didn't properly wait for the current position to be saved to file
Fixed an issue where an empty directory would lead to nothing being tailed

* adding more tests for target.go

* renaming quitComplete -> done per feedback
making PositionsFile private again, no reason for it to be public
removed unnecessary save of empty file on creation of positions file

* fixing lint errors

* cleaning up some comments
removing TODO around directories, added a test to verify behavior
some more cleanup per review

* fixing imports