Convert from a String in the secret test

[jdno]

Stacked on #213, which touches the same tests. GitHub retargets this at main once that one merges.

The test for From<String> on a secret converted from a &str:

#[test]
fn from_string_returns_secret() {
    let secret: TestSecret = "test".into();
}

So it duplicated from_str_returns_secret beside it, and the conversion it was named for went untested. It now converts from a String.

Notes for review

• Verified by deleting impl From<String> from the secret! macro. The test now fails to compile, where before the change neither secret test noticed.
• Only tests/secret.rs changes.

just pre-commit passes.