tests: Set up docker environment for tests

[odscjames]

https://github.com/OpenDataServices/standards-lab/issues/164

[odscjames]

It may seem a bit over the top to set up a whole new compose environment but I've actually rescued this working pull request from my attempt to do #170 and I think this will be really important for #170 (I got that working locally - just not on GH Actions - so I am clear what we need to do here)

Basically, to get #170 to work we need a background worker running. And it needs to run on the same project files data dir as the test runner. We could cram it all in the current setup, but then there would be bits of mess in the code and a developers working files and test files would mix - that really isn't good. Setting up a whole new docker-compose environment now keeps working and test files totally separate and, when we come to do #170, makes adding a worker to the test runner setup pretty easy.

[coveralls]

Pull Request Test Coverage Report for Build 882300120

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 71.34%

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Totals
Change from base Build 881349273:	0.09%
Covered Lines:	458
Relevant Lines:	642

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💛 - Coveralls