RFR: 8377601: JavacTemplateTestBase not reporting javac crashes [v2]

[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.

Chen Liang has updated the pull request incrementally with two additional commits since the last revision:

 - Redundant import
 - Simplified the logic - ct.generate throws already

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29659/files
  - new: https://git.openjdk.org/jdk/pull/29659/files/1267c316..4204f0e1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29659&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29659&range=00-01

  Stats: 19 lines in 1 file changed: 2 ins; 12 del; 5 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