testing: add Output
Output is a method on T, B and F. It provides an io.Writer that writes
to the same test output stream as TB.Log. The new output writer is
used to refactor the implementation of Log. It maintains the formatting
provided by Log while making call site information optional.
Additionally, it provides buffering of log messages. This fixes and
expands on
https://go-review.googlesource.com/c/go/+/646956.
For #59928.
Change-Id: I08179c35a681f601cf125c0f4aeb648bc10c7a9f
GitHub-Last-Rev:
e6e202793c9bc471493187e0556a3a1e7305ff82
GitHub-Pull-Request: golang/go#73703
Reviewed-on: https://go-review.googlesource.com/c/go/+/672395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>