gh-135335: Simplify preload regression test using __main__ (GH-138686)

Simplify preload regression test using `__main__`

With the fix for gh-126631 `__main__` modules can be preloaded and the regression
test for gh-135335 can be simplified to just use a self-contained script rather
than requiring a module.

Note this assumes and implicitly tests that `__main__` is preloaded by default.