Current test of createMuiTheme to deep merges multiple arguments is ineffective

[eedrah]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

I believe the current test to deep merge arguments is ineffective. I am finding that I'm getting errors in this behavior, but before creating an issue with those, I've decided to first fix the test. (There seems to be a lot of code changes around this area anyway, so this might be a more productive way to go. I note that the api went from being undocumented to documented in the last couple of days.)

Expected Behavior 🤔

I'd expect one of the two following mutually exclusive options:

1. It is agreed that the behavior is unwanted, the test is removed, and the functionality is removed from the docs.
2. It is agreed that the behavior is wanted, and the tests are updated. Any failing tests should be fixed shortly after.

My vote is for option 2, and a pull request will follow shortly after this.

[eedrah]

Here's the pull request: #18543