Apply coverage ignore comments as applicable

From: https://pub.dev/packages/coverage

> **Ignore lines from coverage**
> * `// coverage:ignore-line` to ignore one line.
> * `// coverage:ignore-start` and // coverage:ignore-end to ignore range of lines inclusive.
> * `// coverage:ignore-file` to ignore the whole file.

We should apply this to bit of the code that we don't want coverage for.
I'm sure we have a few places that we don't care to test.

**Example**, maybe it's overkill to test `requireSubtleCrypto` logic, we could just make that private method, and use these coverage-ignore comments to avoid counting it in coverage.
https://github.com/google/webcrypto.dart/blob/7a673ab4bdf2c4638e819aa75608e019a3488fb7/lib/src/crypto_subtle.dart#L81-L102

This is really just nice to have and generally users shouldn't hit this. And we don't really want to make testing setups where we can reproduce the behavior. So any testing we do is kind of artificial anyways.