RFR: JDK-8294994: Update Jarsigner and Keytool i18n tests to validate i18n compliance

[Bill Huang]

The jarsigner and keytool are localized into English, German, Japanese and Simplified Chinese. This task is to modify the existing i18n tests to validate i18n compliance in these tools. 

In addition, this task also contains changes for manual test enhancement and simplification which originated from [JDK-8292663](https://bugs.openjdk.org/browse/JDK-8292663.

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Commit messages:
 - Update keytool i18n test.

Changes: https://git.openjdk.org/jdk/pull/10635/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10635&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294994
  Stats: 407 lines in 2 files changed: 395 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10635.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10635/head:pull/10635

PR: https://git.openjdk.org/jdk/pull/10635