Bump proptest from 1.1.0 to 1.2.0 (RustCrypto#242)

Bumps [proptest](https://github.com/proptest-rs/proptest) from 1.1.0 to 1.2.0.
- [Release notes](https://github.com/proptest-rs/proptest/releases)
- [Changelog](https://github.com/proptest-rs/proptest/blob/master/CHANGELOG.md)
- [Commits](proptest-rs/proptest@v1.1.0...v1.2.0)

---
updated-dependencies:
- dependency-name: proptest
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>