Feature request: `allow-unwrap-in-tests` should allow `unwrap_err`

Summary

For best practices, I disallow unwrap to enforce using Result<>.
But in tests, it is necessary to check errors.

given

unwrap_used = "deny"
allow-unwrap-in-tests = "allow"

results in

used `unwrap_err()` on a `Result` value
if this value is an `Ok`, it will panic
consider using `expect_err()` to provide a better panic message
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_used

but expected to be allowed.

Alternative code that doesn't work:

result.err().unwrap().to_string()

# =>  consider using `expect()` to provide a better panic message

result.err().expect("").to_string()

# => called `.err().expect()` on a `Result` value

Lint Name

No response

Reproducer

I tried this code:

<code>

I saw this happen:

<output>

I expected to see this happen:

Version

Additional Labels

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