chainfee: use proper mock for WebAPIFeeSource in tests

lnwallet/chainfee/mocks.go

@@ -0,0 +1,17 @@
+package chainfee
+
+import "github.com/stretchr/testify/mock"
+
+type mockFeeSource struct {
+	mock.Mock
+}
+
+// A compile-time assertion to ensure that mockFeeSource implements the
+// WebAPIFeeSource interface.
+var _ WebAPIFeeSource = (*mockFeeSource)(nil)
+
+func (m *mockFeeSource) GetFeeMap() (map[uint32]uint32, error) {
+	args := m.Called()
+
+	return args.Get(0).(map[uint32]uint32), args.Error(1)
+}