RFR: 8335530: Java file extension missing in AuthenticatorTest

[Daniel Fuchs]

This trivial test was pushed without a .java file extension and none of us spotted this.

This change renames the test to add the proper file extension, converts it to use jupiter APIs instead of TestNG, and fixes a small mistake (s/principal.getName()/principal.getUsername()/).

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Commit messages:
 - typo
 - call assertEquals with parameters in the right order
 - 8335530

Changes: https://git.openjdk.org/jdk/pull/19984/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19984&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335530
  Stats: 120 lines in 2 files changed: 60 ins; 60 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19984.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19984/head:pull/19984

PR: https://git.openjdk.org/jdk/pull/19984