internal/coverage/slicewriter: fix off-by-1 error in seek utilities
The slicewriter Seek method was being too restrictive on offsets
accepted, due to an off-by-one problem in the error checking code.
This fixes the problem and touches up the unit tests.
Change-Id: I75d6121551de19ec9275f0e331810db231db6ea9
Reviewed-on: https://go-review.googlesource.com/c/go/+/488116
Run-TryBot: Than McIntosh <thanm@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>