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.

doc.go

@@ -90,7 +90,7 @@ ConnectHook to get the SQLiteConn.
 					},
 			})
 
-You can also use database/sql.Conn.Raw:
+You can also use database/sql.Conn.Raw (Go >= 1.13):
 
 	conn, err := db.Conn(context.Background())
 	// if err != nil { ... }