Add more content trust tests

Importing from moby's DockerTrustSuite tests.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

internal/test/output/output.go

@@ -0,0 +1,44 @@
+package output
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/pkg/errors"
+)
+
+// Assert checks wether the output contains the specified lines
+func Assert(t *testing.T, actual string, expectedLines map[int]func(string) error) {
+	for i, line := range strings.Split(actual, "\n") {
+		cmp, ok := expectedLines[i]
+		if !ok {
+			continue
+		}
+		if err := cmp(line); err != nil {
+			t.Errorf("line %d: %s", i, err)
+		}
+	}
+	if t.Failed() {
+		t.Log(actual)
+	}
+}
+
+// Prefix returns whether if the line has the specified string as prefix
+func Prefix(expected string) func(string) error {
+	return func(actual string) error {
+		if strings.HasPrefix(actual, expected) {
+			return nil
+		}
+		return errors.Errorf("expected %s to start with %s", actual, expected)
+	}
+}
+
+// Equals returns wether the line is the same as the specified string
+func Equals(expected string) func(string) error {
+	return func(actual string) error {
+		if expected == actual {
+			return nil
+		}
+		return errors.Errorf("got %s, expected %s", actual, expected)
+	}
+}