Create a mock Form for WebTestBase? Is there a way in the testing framework to define a generic form intended only when tests run?

I have created some custom form item Element classes. I have working Unit tests with them, but I would also like to provide Simpletests for them as well.
Core provides several examples of this.  The basic pattern is to create a test module, and have that module provide the form.  In your test ::setUp(), you add the test module to $modules, and then your form should be available for use.

Look through 'core/*/tests/modules' for the test modules, and if you have an IDE that class completes, a lot of the test form are (wait for it) TestForm.

However, in your case, I would look at existing Element classes and see what their integration tests look like.   I suspect you can just emulate what they are doing w/o needing your own test module.