dolphin: add support for union query

Author: aitva

internal/endtoend/testdata/select_union/go/db.go renamed to internal/endtoend/testdata/select_union/mysql/go/db.go

@@ -0,0 +1,12 @@
+{
+  "version": "1",
+  "packages": [
+    {
+      "engine": "mysql",
+      "path": "go",
+      "name": "querytest",
+      "schema": "query.sql",
+      "queries": "query.sql"
+    }
+  ]
+}

internal/endtoend/testdata/select_union/go/models.go renamed to internal/endtoend/testdata/select_union/mysql/go/models.go

@@ -0,0 +1,6 @@
+CREATE TABLE foo (a text, b text);
+
+-- name: SelectUnion :many
+SELECT * FROM foo
+UNION
+SELECT * FROM foo;