RFR: 8183348: Better cleanup for jdk/test/sun/security/pkcs12/P12SecretKey.java

[Mikhail Yankelevich]

* Changed the test to use scratch directory
* Cleaned up the imports

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Commit messages:
 - JDK-8183348: Better cleanup for jdk/test/sun/security/pkcs12/P12SecretKey.java

Changes: https://git.openjdk.org/jdk/pull/24718/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8183348
  Stats: 25 lines in 1 file changed: 1 ins; 6 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/24718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24718/head:pull/24718

PR: https://git.openjdk.org/jdk/pull/24718