RFR: JDK-8294726: Update URLs in minefield tests

[Jonathan Gibbons]

Please review a small change to address a suggestion that was accidentally overlooked in PR #10366

The URLs are changed to use `https://git.openjdk.org/`

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Commit messages:
 - JDK-8294726: Update URLs in minefield tests

Changes: https://git.openjdk.org/jdk/pull/10542/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10542&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294726
  Stats: 6 lines in 6 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10542.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10542/head:pull/10542

PR: https://git.openjdk.org/jdk/pull/10542