Use Self-Hosted Runner for Blockchain Tests

[DavePearce]

The goal here is to enable the xxl self-hosted runner for the blockchain reference tests. The existing issue appears to be something to do with running out of memory. It has been noted that the self-hosted runners use a different (presumably later) version of Gradle.

Questions:

• What version of gradle is running on the self-hosted runners?
• Is it gradle itself which is running out of memory (i.e. rather than the tests)?
• Can we recreate this issue on our local machines?

NOTE: also this line in gradle.properties which is limiting the amount of memory available to gradle itself:

org.gradle.jvmargs=-Xmx4096m