Make test client accept environ_base from flask_test_client

[vrt-boss-bfx]

Flask's testing.TestClient sets and allows overriding its environ_base.

This PR makes flask_socketio's SocketIOTestClient honor the environ_base if it exists.

This would allow testing handler that depend on overridden environ_base. For example

import unittest

from flask import Flask, request
from flask_socketio import SocketIO

app = Flask(__name__)
app.config["SECRET_KEY"] = "secret"
socketio = SocketIO(app)


@socketio.on("connect")
def on_connect(auth):
    return request.remote_addr == "10.0.0.0"


class TestWithClient(unittest.TestCase):
    def test_connect(self):
        flask_test_client = app.test_client()
        flask_test_client.environ_base = {
            **flask_test_client.environ_base,
            "REMOTE_ADDR": "10.0.0.0",
        }

        client = socketio.test_client(app, flask_test_client=flask_test_client)
        self.assertTrue(client.is_connected())