fix to use MustParse("xxx") instead of Must(Parse("xxxx"))

[kotaroyamazaki]

Must(Parse("xxxx")) is considered legacy code to forced uuid to parse because the code is pre-implementation of Must(Parase("xxxx")) (https://github.com/google/uuid/pull/26)

This one is simpler.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[noahdietz]

Hello! If you are still interested in making this contribution, please follow the instructions above to sign the CLA. Then, also change the instances of Must(Parse( in tests to MustParse as well, please.

If you are not interested in continuing with this PR, do not respond in a few working days, or do not want to sign the Google CLA, I will make the change myself. Thanks.

[bormanp]

Please address the failed check about commit messages. Other than that I see no problem with this code.

[kotaroyamazaki]

@noahdietz @bormanp Sorry for the delay. I've fixed the CI error and also updated the test code.