runtime/coverage: recognise Plan 9 error message in emitToNonexistentDir
In TestCoverageApis/emitToNonexistentDir there is a list of error
messages to match when a nonexistent directory is opened. The list
has message text only for Unix and Windows. Add the corresponding
text for Plan 9.
Fixes #55983
Change-Id: Id32130300cb02394b319e1aeb1229ee147b4afb2
Reviewed-on: https://go-review.googlesource.com/c/go/+/437557 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Than McIntosh <thanm@google.com>