Make easy improvements to our e2e test process

[sodic]

This issue covers several e2e test-related improvements:

• We recently documented how a PR author should run e2e tests after getting the approval on the PR, but we should add more emphasis to this. One argument we forgot to mention is that, when you run e2e tests before the PR finishes all its iterations, you're less likely to check whether their final form is OK.
• We should make it clear that the PR author is the one responsible for checking e2e test outputs (not just updating them without looking). Enforcing this will help with the first point.
• We should stop checking some generated files, mostly those in dist directories. It creates a lot of noise and only tests the TS compiler. For example, one foo.ts file change changes 7 files in a single e2e test (files.manifest, .waspchecksums, foo.ts, dist/foo.js, dist/foo.js.map, dist/foo.d.ts, dist/foo.d.ts.map), and we don't care about most of them.