Fix client tests to work on Windows.

- Powershell is no longer available in nanoserver, so change commands to
  run accordingly.
- Set platform specific commands for short and long running containers
- Skips 2 tests which do not run on Windows.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

client_unix_test.go

@@ -32,7 +32,9 @@ const (
 )
 
 var (
-	testImage string
+	testImage    string
+	shortCommand = withProcessArgs("true")
+	longCommand  = withProcessArgs("/bin/sh", "-c", "while true; do sleep 1; done")
 )
 
 func init() {