channeldb: inject clock into database

Use our standard clock mock for database time queries.

channeldb/invoice_test.go

@@ -304,7 +304,6 @@ func TestInvoiceAddTimeSeries(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable to make test db: %v", err)
 	}
-	db.Now = func() time.Time { return testNow }
 
 	// We'll start off by creating 20 random invoices, and inserting them
 	// into the database.
@@ -556,7 +555,6 @@ func TestDuplicateSettleInvoice(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable to make test db: %v", err)
 	}
-	db.Now = func() time.Time { return testNow }
 
 	// We'll start out by creating an invoice and writing it to the DB.
 	amt := lnwire.NewMSatFromSatoshis(1000)
@@ -631,7 +629,6 @@ func TestQueryInvoices(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable to make test db: %v", err)
 	}
-	db.Now = func() time.Time { return testNow }
 
 	// To begin the test, we'll add 50 invoices to the database. We'll
 	// assume that the index of the invoice within the database is the same