[DependencyInjection] Test constants
merged branch gnugat/dic-test-constants (PR #8663)

This PR was squashed before being merged into the master branch (closes #8663).

Discussion
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[DependencyInjection] Test constants

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | **yes**
| License       | MIT

Added a test for constant support in XML configuration files.
Related PR: #8661

Commits
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9acedb7 [DependencyInjection] Test constants