[jdk21u-dev] RFR: 8328301: Convert Applet test ManualHTMLDataFlavorTest.java to main program

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle

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Commit messages:
 - Backport f7f291c5d4d2d01dab3ccda7518ebc13f6bd58f6

Changes: https://git.openjdk.org/jdk21u-dev/pull/1641/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1641&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328301
  Stats: 260 lines in 2 files changed: 15 ins; 189 del; 56 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1641.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1641/head:pull/1641

PR: https://git.openjdk.org/jdk21u-dev/pull/1641