[FrameworkBundle] Added tests for the DelegatingEngine.
[FrameworkBundle] Made DelegatingEngine::getEngine() public.
merged branch jakzal/templating-delegating-engine (PR #8280)

This PR was merged into the master branch.

Discussion
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[Templating|FrameworkBundle] Made DelegatingEngine::getEngine() public

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7254
| License       | MIT
| Doc PR        | -

As a bonus I've covered both classes with tests.

Commits
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a54cbff [FrameworkBundle] Made DelegatingEngine::getEngine() public.
3f84cd3 [Templating] Made DelegatingEngine::getEngine() public.
0a72a99 [FrameworkBundle] Added tests for the DelegatingEngine.
6c31ab2 [Templating] Added tests for the DelegatingEngine.