lnmock: add new package lnmock to host mocks

This commit introduces a new package to host mocked objects that can be
used for all the unit tests.

lnmock/clock.go

@@ -0,0 +1,30 @@
+// NOTE: forcetypeassert is skipped for the mock because the test would fail if
+// the returned value doesn't match the type.
+package lnmock
+
+import (
+	"time"
+
+	"github.com/lightningnetwork/lnd/clock"
+	"github.com/stretchr/testify/mock"
+)
+
+// MockClock implements the `clock.Clock` interface.
+type MockClock struct {
+	mock.Mock
+}
+
+// Compile time assertion that MockClock implements clock.Clock.
+var _ clock.Clock = (*MockClock)(nil)
+
+func (m *MockClock) Now() time.Time {
+	args := m.Called()
+
+	return args.Get(0).(time.Time)
+}
+
+func (m *MockClock) TickAfter(d time.Duration) <-chan time.Time {
+	args := m.Called(d)
+
+	return args.Get(0).(chan time.Time)
+}