Fix test on Windows

https://ci.appveyor.com/project/otoolep/rqlite/builds/48999854

db/swappable_db.go

@@ -25,7 +25,9 @@ func OpenSwappable(dbPath string, fkEnabled, wal bool) (*SwappableDB, error) {
 	return &SwappableDB{db: db}, nil
 }
 
-// Swap swaps the underlying database with that at the given path.
+// Swap swaps the underlying database with that at the given path. The Swap operation
+// may fail on some platforms if the file at path is open by another process. It is
+// the caller's responsibility to ensure the file at path is not in use.
 func (s *SwappableDB) Swap(path string, fkConstraints, walEnabled bool) error {
 	if !IsValidSQLiteFile(path) {
 		return fmt.Errorf("invalid SQLite data")

db/swappable_db_test.go

@@ -75,6 +75,9 @@ func Test_SwapSuccess(t *testing.T) {
 	defer swappableDB.Close()
 
 	// Perform the swap
+	if err := srcDB.Close(); err != nil {
+		t.Fatalf("failed to close source database pre-swap: %s", err)
+	}
 	if err := swappableDB.Swap(srcPath, false, false); err != nil {
 		t.Fatalf("failed to swap database: %s", err)
 	}