sqlite3_test.go: Move Go 1.13 test to sqlite3_go113_test.go (#883)

Commit 4f7abea96e added a test that uses Conn.Raw, which was added in
Go >= 1.13. The go-sqlite3 project runs tests with Go >= 1.11. Remove
the test from sqlite3_test.go, so it only runs with the correct
versions of Go.

Instead of adding a new test, modify the existing test that already
uses Conn.Raw() to check the type of driverConn.

sqlite3_go113_test.go

@@ -49,6 +49,10 @@ func TestBeginTxCancel(t *testing.T) {
 				if !ok {
 					t.Fatal("unexpected: wrong type")
 				}
+				// checks that conn.Raw can be used to get *SQLiteConn
+				if _, ok = driverConn.(*SQLiteConn); !ok {
+					t.Fatalf("conn.Raw() driverConn type=%T, expected *SQLiteConn", driverConn)
+				}
 
 				go cancel() // make it cancel concurrently with exec("BEGIN");
 				tx, err := d.BeginTx(ctx, driver.TxOptions{})