RFR: 8307732: build-test-lib is broken

[Johannes Bechberger]

Fixes the issue by adding `--add-exports` for the required modules and making the `HttpHeaderParser` final (fixing an issue with calling overridable methods in a constructor).

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Commit messages:
 - Fix make build-test-lib

Changes: https://git.openjdk.org/jdk/pull/13885/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13885&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307732
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13885.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13885/head:pull/13885

PR: https://git.openjdk.org/jdk/pull/13885