Enable pass on non-linux systems

There is nothing linux specific about pass. In particular, I use it in
openbsd.

Signed-off-by: Rafael Ávila de Espíndola <rafael@espindo.la>

pass/pass_test.go

@@ -0,0 +1,75 @@
+package pass
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/docker/docker-credential-helpers/credentials"
+)
+
+func TestPassHelper(t *testing.T) {
+	helper := Pass{}
+
+	creds := &credentials.Credentials{
+		ServerURL: "https://foobar.docker.io:2376/v1",
+		Username:  "nothing",
+		Secret:    "isthebestmeshuggahalbum",
+	}
+
+	helper.Add(creds)
+
+	creds.ServerURL = "https://foobar.docker.io:9999/v2"
+	helper.Add(creds)
+
+	credsList, err := helper.List()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for server, username := range credsList {
+		if !(strings.Contains(server, "2376") ||
+			strings.Contains(server, "9999")) {
+			t.Fatalf("invalid url: %s", creds.ServerURL)
+		}
+
+		if username != "nothing" {
+			t.Fatalf("invalid username: %v", username)
+		}
+
+		u, s, err := helper.Get(server)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if u != username {
+			t.Fatalf("invalid username %s", u)
+		}
+
+		if s != "isthebestmeshuggahalbum" {
+			t.Fatalf("invalid secret: %s", s)
+		}
+
+		err = helper.Delete(server)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		username, _, err = helper.Get(server)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if username != "" {
+			t.Fatalf("%s shouldn't exist any more", username)
+		}
+	}
+
+	credsList, err = helper.List()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(credsList) != 0 {
+		t.Fatal("didn't delete all creds?")
+	}
+}