cmd/api: skip tests on platforms that cannot exec

cmd/api: skip tests on platforms that cannot exec

These tests execute commands using exec.Command.

Change-Id: I2708d6d24762fe2b2a902b1b221cc67392bf5c41
Reviewed-on: https://go-review.googlesource.com/c/go/+/435235
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>