Turns out relying on PWTRAP in stack is not reliable: depending on the
call structure, the stack might be cut unpredictably by Node.js.
This patch removes PWTRAP and instead plumbs explicit stack and
pre-set `apiName` all the way down to `wrapApiCall`.
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)
