Executing tests in Docker is broken

[captaincoordinates]

The README's instructions for executing tests in Docker are broken. This is partly because tests/execute-tests.sh was deleted (seemingly without reason) in this commit and partly because the approach has not been maintained in line with new test dependencies.

Personally I would prefer that the GitHub Actions Workflow executed tests using Docker, so that developers can guarantee tests will behave equally in both environments and as an incentive to ensure the Docker approach is maintained, however I appreciate that approach might not be to everyone's taste.

I have a fork that fixes Docker tests and I will submit a PR after creating this issue.

PR #104