Add a new function runtime/coverage.snapshot(), which samples the
current values of coverage counters in a running "go test -cover"
binary and returns percentage of statements executed so far. This
function is intended to be used by the function testing.Coverage().
Updates #59590.
Change-Id: I861393701c0cef47b4980aec14331168a9e64e8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/495449
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>