RFR: 7902924: Add build script for jtreg plugin

[Maurizio Cimadamore]

This patch adds support for building the jtreg plugin with gradle.

This is a much more stable way to build the plugin, as the IntelliJ build dependency is automatically downloaded by gradle.

The build relies on the IntelliJ support for gradle-ized plugins:

https://github.com/JetBrains/gradle-intellij-plugin

Which is officially supported by JetBrains, see:

https://plugins.jetbrains.com/docs/intellij/gradle-build-system.html


The only thing that needs to be done by developers before firing up a gradle build, is to specify the path of a jtreg root in the gradle.properties file. Everything else should work out of the box.

I have also updated build instructions to reflect the new build system.

Please let me know if this is an improvement over the build steps we had before (I think it is, but I have not tested this extensively).

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Commit messages:
 - Revert jcheck changes
 - Revert changes to jcheck conf
 - Ignore gradlew executable in jcheck
 - Merge branch 'master' into gradle-build-plugin
 - Tweak build script
 - Initial push
 - Merge branch 'master' into jtregfix_2021_v11
 - Remove @Override which caused compilation failure in latest Intellij

Changes: https://git.openjdk.java.net/jtreg/pull/9/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=9&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7902924
  Stats: 462 lines in 44 files changed: 362 ins; 83 del; 17 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/9.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/9/head:pull/9

PR: https://git.openjdk.java.net/jtreg/pull/9