feat(api): add explicit async testInfo.attach
We add an explicit async API for attaching file paths (and Buffers) to
tests that can be awaited to help users ensure they are attaching files
that actually exist at both the time of the invocation and later when
reporters (like the HTML Reporter) run and package up test artifacts.
This is intended to help surface attachment issues as soon as possible
so you aren't silently left with a missing attachment
minutes/days/months later when you go to debug a suddenly breaking test
expecting an attachment to be there.
NB: The current implemntation incurs an extra file copy compared to
manipulating the raw attachments array. If users encounter performance
issues because of this, we can consider an option parameter that uses
rename under the hood instead of copy. However, that would need to be
used with care if the file were to be accessed later in the test.
