RFR: 8377601: JavacTemplateTestBase not reporting javac crashes

[Chen Liang]

JavacTemplateTestBase does not fail if javac crashes. A javac crash is not reflected in the diagnostics, but rather printed in System.err, which can be redirected. We should redirect that stream and ensure that stream is empty for our test cases.

This test framework bug is discovered during project valhalla bworld prototype development.

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Commit messages:
 - Ensure there is no printed error like crash from compilation test

Changes: https://git.openjdk.org/jdk/pull/29659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377601
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29659/head:pull/29659

PR: https://git.openjdk.org/jdk/pull/29659