[release-branch.go1.15] internal/execabs: only run tests on platforms that support them
Fixes #43793
Change-Id: I3bf022a28b194f0089ea96d93e56bbd9fb7e0aa8
Reviewed-on: https://go-review.googlesource.com/c/go/+/285056
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>