Flask Unittest for Post Method

I am writing a Flask unit test for a function that would return a render template. I tried few ways but it seems not working. Here is the function:

@app.route('/', methods=['POST'])
@lti(request='initial', error=error, app=app)
def chooser(lti=lti):
   return_url = request.form.get('launch_presentation_return_url', '#')
   return render_template(
       'chooser.html'
   )

Few ways that I have been trying:

# 1st way    
rv = self.app.post('/')
self.assertTrue('Choose an Icon to Insert' in rv.get_data(as_text=True))
# Error
self.assertTrue('Choose an Icon to Insert' in rv.get_data(as_text=True))
AssertionError: False is not true


# 2nd way    
rv = self.app.post('/chooser.html')
assert '<h1>Choose an Icon to Insert</h1>' in rv.data
# Error
assert 'Choose an Icon to Insert' in rv.data
AssertionError

chooser.html

 <body>
    <h1>Choose an Icon to Insert</h1>
 </body>

Thanks for all your helps.