Thread safety when interacting with system test deployment in system tests

[patchwork01]

Background

Split from:

• Parallelise system tests #1794

Description

We'd like to ensure that any interactions with the system test cluster and associated resources are done in a thread safe way.

Analysis

The system test cluster and associated resources are currently deployed in SleeperSystemTestExtension.beforeAll. This is actually called many times, once for each test class. We can either make that thread safe, or move it to the initialisation of SystemTestDeploymentContext.

The SystemTestDeploymentContext is held in a static constant for each test type (in memory, LocalStack or deployed), so if we don't mutate anything after that is constructed, it will be thread safe.

We can adjust DeployedSystemTestResources to avoid any mutable fields on the class.