Insert all and only enabled (and not skipped) engines in default value of RENDER_ENGINE_VALUES

## Desired behavior
I would like that after a configuration of ign-rendering on a given system, `ctest` would actually test just and only the enabled engines. At the moment this is not the case, as the list of tested engines is hardcoded in https://github.com/ignitionrobotics/ign-rendering/blob/50e1e5fa936176a570b6b4cccbbe081334af6057/test/test_config.h.in#L14

## Alternatives considered
Leave the situation like this, where the default value of RENDER_ENGINE_VALUES reflect the official OpenRobotics CI configuration. In that case however it would be great if the test could explicitly fail on missing engine, so that it is clear that the "user" (tipically someone packaging the library) needs to set the `RENDER_ENGINE_VALUES` env variable value.

## Implementation suggestion
If the feature is welcome, I can look in the suggestion.

## Additional context
See https://github.com/conda-forge/libignition-rendering4-feedstock/pull/19#issuecomment-937525058 for the context in which this issue was detected.

Start with test/test_config.h.in at the hardcoded RENDER_ENGINE_VALUES definition, then inspect how ign-rendering configuration exposes enabled and skipped engines to ctest. Confirm the default list reflects the configured engines and verify behavior with the test configuration described in the issue; done means ctest runs only enabled, non-skipped engines.

Written by the indexing model from the issue text.