241: Rename test-environment variable

Fixes #241

[x] Is the title of the Pull Request correct?
[x] Is the title of the corresponding issue correct?
[x] Have you updated the changelog?
[x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
[x] Are you mentioning the issue which this PullRequest fixes ("Fixes...")
[x] Before you merge don't forget to run all tests for all Exasol version, by adding [run all tests] to the commit message
[x] Are the CLI usage examples up to date?

exasol / integration-test-docker-environment