Run the build test suite against the incubator javac branch

[fbricon]

We want to gauge how the javac support work improves over time over at https://github.com/eclipse-jdtls/eclipse-jdt-core-incubator/tree/dom-with-javac. To do so, we need to run the test suite against #3123

[fbricon]

See how vscode-java builds jdt.ls against the javac branch: https://github.com/fbricon/vscode-java/blob/javac-build/.github/workflows/javac-upload.yml

We'll prolly need to actually publish the javac update site first.
Tests would need to be run with specific flags though (in a maven plugin, set in a maven profile).

[mickaelistria]

The javac-based JDT repo is available for consumption/testing at https://ci.eclipse.org/ls/job/jdt-core-incubator/job/dom-with-javac/lastSuccessfulBuild/artifact/repository/target/repository/