test(invocation): add command.Covers and subject consistency tests

Also improve the maintainability of the tests by a) providing a set of fixed Personas
and then generating a slew of valid delegation tokens, invalid delegation tokens
and proof-chains thereof.

pkg/policy/policytest/policy.go

@@ -0,0 +1,20 @@
+// Package policytest provides values and functions that are useful when
+// testing code that relies on Policies.
+package policytest
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/ucan-wg/go-ucan/pkg/policy"
+)
+
+// EmptyPolicy provides a policy with no statements for testing purposes.
+func EmptyPolicy(t *testing.T) policy.Policy {
+	t.Helper()
+
+	pol, err := policy.FromDagJson("[]")
+	require.NoError(t, err)
+
+	return pol
+}