Fix a test corruption issue, where a test accidentally unsets
the `NOMAD_LICENSE` environment variable, that's relied on by some
tests.
As a habit, tests should always restore the environment variable value
on test completion. Golang 1.17 introduced
[`t.Setenv`](https://pkg.go.dev/testing#T.Setenv) to address this issue.
However, as 1.0.x and 1.1.x branches target golang 1.15 and 1.16, I
opted to use a helper function to ease backports.
