Suppress warnings when cargo test --no-default-features

[giraffate]

Suppress warnings like below:
https://github.com/Keats/jsonwebtoken/actions/runs/3997872968/jobs/6859764490#step:6:11

warning: unused imports: `Header`, `Validation`, `decode`, `encode`
Warning:  --> tests/ecdsa/mod.rs:3:5
  |
3 |     decode, encode, Algorithm, DecodingKey, EncodingKey, Header, Validation,
  |     ^^^^^^  ^^^^^^                                       ^^^^^^  ^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused import: `time::OffsetDateTime`
Warning:  --> tests/ecdsa/mod.rs:6:5
  |
6 | use time::OffsetDateTime;
  |     ^^^^^^^^^^^^^^^^^^^^

[Keats]

Thanks!