Remove the index signature from TestContext

[dfreeman]

The TestContext type has a [key: string]: unknown index signature inherited from BaseContext. This has led to some confusion, as it muddies the error message folks see when they try to access fields that were present in the DT types but not the native ones, like element.

It also doesn't mesh super well with how folks tend to add custom properties to the context type by explicitly declaring them and then setting the this type appropriately for their tests. Ultimately we want for that pattern to go away entirely with <template>, of course, but in the meantime it would be nice to avoid extra confusion.

The index signature on BaseContext doesn't actually appear to be required, and ultimately the way we're using the type is equivalent to just object. Accordingly, I'm making BaseContext an alias for object here and otherwise attempting to touch the code as little as possible. The bits that did have to change were largely places we were implicitly taking advantage of the index signature to skirt typechecking.

[chriskrycho]

For a possible follow-on (keeping type changes separate from behavioral changes):

return (
  'pauseTest' in context && 
  typeof maybeContext['pauseTest'] === 'function' &&
  'resumTest' in context && 
  typeof maybeContext['resumeTest'] === 'function'
);

[dfreeman]

I had that and my editor was happy with it, but it turns out the in narrowing for object landed in TS 4.9, and definitely didn't want to touch that as part of this 😅