Also test with Groovy 2.5 on classpath for Gradle 4, 5 & 6

[timtebeek]

What's changed?

Add an integration test source set using Groovy 2.5.x, and unit tests where we've seen failures on that version.

What's your motivation?

Make it easier detect & troubleshoot parser issues specific to Groovy 2.

Anything in particular you'd like reviewers to focus on?

Wasn't sure on the Java version required for this source set, since Java 16+ requires Groovy 3+.

Have you considered any alternatives or workarounds?

I'd also tried a separate rewrite-groovy-test module, which makes it easier to also set a lower required Java version, but didn't like adding yet another top level project if we can avoid it. Getting mixed results locally and having a bit of trouble with IDE. Figured open a draft to see results in CI.

Any additional context

We've seen an uptick in parser issues reported following some recent-ish changes

• b1cb470
• 8ef710d?diff=unified&w=1
• 78ca6ad

[timtebeek]

This is getting frustrating; in an earlier iteration I'd seen these fail, but somehow it's not replicating right now. 🤔