Integration test for docker-compose.storage.s3.yml

We did remove by mistake docker-compose.storage.s3.yml, which is used in some community installations to provide Mender storage via S3 bucket. The removal affected 2.3 and 2.4 releases, but it was restored afterwards.

We need a test to use this file, otherwise there is a risk that 1) is not maintained and/or 2) gets eliminated again by mistake.

The old test can be taken as inspiration.

References:

• PR restoring the file: https://github.com/mendersoftware/integration/pull/923
• Commit removing the file (and the original test): https://github.com/mendersoftware/integration/commit/bc39515d38992feb7514f3c570a137143e02a4c9

Acceptance criteria:

• There is an integration test for it that runs in CI