[jdk21u-dev] RFR: 8304020: Speed up test/jdk/java/util/zip/ZipFile/TestTooManyEntries.java and clarify its purpose

[Goetz Lindenmaier]

I backport this for parity with 21.0.3-oracle.

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Commit messages:
 - Backport f0b7eb519ae1cf6f4cb2baea55b5e02a46a8192e

Changes: https://git.openjdk.org/jdk21u-dev/pull/172/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304020
  Stats: 316 lines in 3 files changed: 226 ins; 90 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/172/head:pull/172

PR: https://git.openjdk.org/jdk21u-dev/pull/172