add tests for deduceConstraint

Signed-off-by: Jess Frazelle <acidburn@google.com>

ensure_test.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/sdboyer/gps"
 )
 
 func TestEnsureOverrides(t *testing.T) {
@@ -88,6 +90,26 @@ func main() {
 	}
 }
 
+func TestDeduceConstraint(t *testing.T) {
+	sv, err := gps.NewSemverConstraint("v1.2.3")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	constraints := map[string]gps.Constraint{
+		"v1.2.3": sv,
+		"5b3352dc16517996fb951394bcbbe913a2a616e3":      gps.Revision("5b3352dc16517996fb951394bcbbe913a2a616e3"),
+		"g4@golang.org-20161116211307-wiuilyamo9ian0m7": gps.NewVersion("g4@golang.org-20161116211307-wiuilyamo9ian0m7"),
+	}
+
+	for str, expected := range constraints {
+		c := deduceConstraint(str)
+		if c != expected {
+			t.Fatalf("expected: %#v, got %#v for %s", expected, c, str)
+		}
+	}
+}
+
 func TestCopyFolder(t *testing.T) {
 	dir, err := ioutil.TempDir("", "dep")
 	if err != nil {