Create a mock Form for WebTestBase?
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.