RFR: 8342704: GHA: Report truncation is broken after JDK-8341424

[Aleksey Shipilev]

When GH output is too large, we do not actually truncate it after [JDK-8341424](https://bugs.openjdk.org/browse/JDK-8341424). The error log would be:


Run bash ./.github/scripts/gen-test-results.sh "$GITHUB_STEP_SUMMARY"
./.github/scripts/gen-test-results.sh: line 28: report-utils.sh: No such file or directory
./.github/scripts/gen-test-results.sh: line 97: truncate_summary: command not found

Error: Process completed with exit code 127.
Error: $GITHUB_STEP_SUMMARY upload aborted, supports content up to a size of 1024k, got 1579k. For more information see: https://docs.github.com/actions/using-workflows/workflow-commands-for-github-actions#adding-a-markdown-summary


This is because we import report-utils.sh incorrectly: scripts try to find it in current directory, which is not the same directory where the importing scripts are located. And we never see it in normal cases, because scripts are just eating the errors without any other observable effects.

Additional testing: 
  - [x] GHA checks work with deliberately failing tests

-------------

Commit messages:
 - Revert test hook
 - Test hook
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21617/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21617&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342704
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21617.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21617/head:pull/21617

PR: https://git.openjdk.org/jdk/pull/21617