[release-branch.go1.13] cmd/doc: skip failing TestDotSlashLookup on Windows

This test was fixed by changing cmd/doc behavior in CL 204442.

Backporting that non-test code change is unlikely to be appropriate
this late in Go 1.13 release cycle. A failing test can cover up other
regressions, so skip this known failing test to fix the builder.

For #35236.
For #36181.

Change-Id: I07e795e75d7e37bc96ab68607d5d5cc9254342f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/223780


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>