Fix staticcheck issues

[ashmckenzie]

This PR fixes the following go-staticcheck errors:

$ staticcheck
entry_test.go:39:49: should not use built-in type string as key for value; define your own type to avoid collisions (SA1029)
entry_test.go:59:50: should not use built-in type string as key for value; define your own type to avoid collisions (SA1029)
entry_test.go:63:50: should not use built-in type string as key for value; define your own type to avoid collisions (SA1029)
logrus_test.go:547:2: should merge variable declaration with assignment on next line (S1021)

[ashmckenzie]

@sirupsen here's a very small PR to fix some go-staticcheck issues in a couple of tests 🙂

[sirupsen]

Cheers