cmd/dist: handle -json flag in runPending (clean up)

cmd/dist: handle -json flag in runPending (clean up)

Document work fields a bit more, and move code that
synthesizes JSON-encoded skip events to testjson.go.

For #37486.
For #61557.

Change-Id: Iffc23cf990bc39696e1e3fce8ce5a6790fc44e78
Reviewed-on: https://go-review.googlesource.com/c/go/+/512115
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>