ssh/terminal: run tests only on supported platforms

The tag matches the platforms defined in util*.go
where the most tested logic is defined.

Change-Id: I90f67d988c795738c3effbc8554a933a7cb355d2
Reviewed-on: https://go-review.googlesource.com/112555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed