build: Ignore missing features in tests crates

[caspermeijn]

Only the crate tests-2015 uses the feature edition-2015, but the code is shared with the tests crate.

Same for tests-no-std which disables feature std, but reuses the code with the tests crate.

Therefore the tests, tests-2018 and tests-no-std crates should be configured to ignore the lint about missing features.

Cargo clippy reports:

warning: unexpected `cfg` condition value: `edition-2015`
 --> tests-no-std/../tests/src/build.rs:5:14
  |
5 |     if #[cfg(feature = "edition-2015")] {
  |              ^^^^^^^-----------------
  |              |
  |              help: remove the condition
  |
  = note: no expected values for `feature`
  = help: consider adding `edition-2015` as a feature in `Cargo.toml`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default