Track deployed instances separately from instance and tables of a system test

[patchwork01]

Background

Split from:

• System test DSL to work with multiple tables #1842

Also relevant to:

• Parallelise system tests #1794

Description

We'd like to separate the context of which instances are deployed while system tests are running, from the context of which tables are associated with the running test, and which instance or table is currently active for the test.

We'd like this to make it easier to run multiple tests in parallel using different tables in the same instance. We'd also like this to make it easier to add and work with tables during a system test.

Analysis

There are a lot of methods in SleeperInstanceContext and SystemTestTablesContext which handle working with the currently active instance or table. We can move these to a separate class or classes which are responsible for tracking the current instance and table.