SDC `ComponentRenderTest` is error-prone

Problem/Motivation

`ComponentRenderTest` contains lots of "test" (quotes intended) methods.

For testing performance reasons, this didn't use proper test methods when included in core.
Since then we've improved our testing performance by a lot.

And the real issue is, because of how phpunit handles exceptions, this might be hiding bugs/BC breaks.

E.g. checkInvalidSlot includes expectException. This means that once the exception is triggered, the test passes. But then checkEmptyProps never runs!

Steps to reproduce

See MR !13117 which is passing but should be failing.

Proposed resolution

Each `check` method should be a proper isolated test method.

Remaining tasks

• ✅ MR with proof (!13117).
• ✅ MR with fix (!13118).

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.