Unit test for WithContentTrustFromEnv

I authored this for `contentTrustEnabled` prior to 7f207f3f95, so this now
tests the funcation argument version.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli/command/cli_options_test.go

@@ -0,0 +1,37 @@
+package command
+
+import (
+	"os"
+	"testing"
+
+	"gotest.tools/assert"
+)
+
+func contentTrustEnabled(t *testing.T) bool {
+	var cli DockerCli
+	assert.NilError(t, WithContentTrustFromEnv()(&cli))
+	return cli.contentTrust
+}
+
+// NB: Do not t.Parallel() this test -- it messes with the process environment.
+func TestWithContentTrustFromEnv(t *testing.T) {
+	envvar := "DOCKER_CONTENT_TRUST"
+	if orig, ok := os.LookupEnv(envvar); ok {
+		defer func() {
+			os.Setenv(envvar, orig)
+		}()
+	} else {
+		defer func() {
+			os.Unsetenv(envvar)
+		}()
+	}
+
+	os.Setenv(envvar, "true")
+	assert.Assert(t, contentTrustEnabled(t))
+	os.Setenv(envvar, "false")
+	assert.Assert(t, !contentTrustEnabled(t))
+	os.Setenv(envvar, "invalid")
+	assert.Assert(t, contentTrustEnabled(t))
+	os.Unsetenv(envvar)
+	assert.Assert(t, !contentTrustEnabled(t))
+}