This patch:
- adds call logs to track screenshot timeouts, e.g. due to
waiting for web fonts
- makes sure all snapshot expectations have `.png` extension
- throws a polite error when given a buffer or a string instead of a
page or a locator
- removes stray NL between error description and call log
- makes sure `apiName` is always correct (and adds a test for it)
Makes it easier to understand that expect does indeed have a separate timeout.
```
Error: expect(received).toHaveCount(expected) // deep equality
Expected: 0
Received: 1
Call log:
- expect.toHaveCount with timeout 500ms
- waiting for selector "span"
- selector resolved to 1 element
- unexpected value "1"
- selector resolved to 1 element
- unexpected value "1"
- selector resolved to 1 element
- unexpected value "1"
```
