RFR: Upgrade JUnit 5

[Christian Stein via github.com]

The pull request has been updated with additional changes.

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Added commits:
 - 732b8f71:	Update README.md

Pull request:
https://git.openjdk.java.net/skara/pull/66

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/66/webrev.01
 - inc: https://webrevs.openjdk.java.net/skara/66/webrev.00-01

Updated full patch:
https://git.openjdk.java.net/skara/pull/66.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/66/head:pull/66