internal/coverage: implement conforming Seek method in slicereader
Implement a real Seek() method in the slicereader helper (prior to
this it had a simplified SeekTo function), so that slicereader's will
satisfy the ReadSeeker interface (needed in a subsequent patch).
Change-Id: I832e3ec1e34d0f8c6b5edf390470f6f943c6ece0
Reviewed-on: https://go-review.googlesource.com/c/go/+/495438
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>