cli/command/stack: TestConfigMergeInterpolation: various fixes
cli/command/stack: TestConfigMergeInterpolation: various fixes

- Make the package-level configMergeTests local to the test itself.
- Rename fields to better describe intent
- Remove some redundant variables
- Reverse "expected" and "actual" fields for consistency
- Use assert.Check() to not fail early

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,docker/cli,thaJeztah/cli,thaJeztah/cli