`nimble test` gives warnings about unused imports
I just ran into this with `nimble test`:
```
$ choosenim show
  Selected: 1.2.0
   Channel: stable

$ nimble --version
nimble v0.11.4 compiled at 2020-05-30 16:46:40
git hash: 63695f490728e3935692c29f3d71944d83bb1e83

$ nimble test
  Executing task test in /home/schwa/sd/nim/lazytree/lazytree.nimble
/home/schwa/sd/nim/lazytree/lazytree_31480.nims(1, 8) Warning: imported and not used: 'system' [UnusedImport]
/home/schwa/sd/nim/lazytree/lazytree_31480.nims(5, 22) Warning: imported and not used: 'strutils' [UnusedImport]
...
```

_Originally posted by @sschwarzer in https://github.com/nim-lang/nimble/issues/706#issuecomment-636357810_