[skara] RFR: build: run build and tests in parallel

[Erik Helin]

Hi all,

please review this patch that enables builds and tests to happen in parallel. I also tweaked the build a little:

- the Gradle daemon now gets an `Xmx` of `2048 Mb`
- Gradle will run up to 100 tests in the same JVM before forking a new one
- the JVM for compiling will be forked, reducing heap pressure on the Gradle daemon thread
- enabled the file system watching feature for faster rebuilds

The results on my AMD Ryzen 5 3600 is that `sh gradlew clean && sh gradlew --info test` went from ~10 minutes to ~2 minutes :partying_face: �� 

Testing:
- [x] `make images` on Linux x64
- [x] `make test` on Linux x64

Thanks,
Erik

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

Depends on: https://git.openjdk.java.net/skara/pull/1008

Commit messages:
 - Merge master
 - Update BotSlackHandlerTest timeout
 - build: run build and tests in parallel

Changes: https://git.openjdk.java.net/skara/pull/1009/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1009&range=00
  Stats: 21 lines in 3 files changed: 20 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1009.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1009/head:pull/1009

PR: https://git.openjdk.java.net/skara/pull/1009