input: Write a function to create an application using Python and Flask that submits form data (Name, Email, Phone) and stores it in a SQLite database. The function should have an assertion test to check the functionality. If the form is submitted successfully, the function should return 'Success'. Otherwise, it should return 'Failure'.
  
  assert submit_form()=='Success'
  assert submit_form()=='Failure'
output: def submit_form():
      from flask import Flask, request, render_template_string
      import sqlite3
  
      app = Flask(__name__)
  
      @app.route('/submit', methods=['POST'])
      def submit():
          name = request.form['name']
          email = request.form['email']
          phone = request.form['phone']
  
          conn = sqlite3.connect('form_data.db')
          cursor = conn.cursor()
          cursor.execute('''CREATE TABLE IF NOT EXISTS submissions (
                              id INTEGER PRIMARY KEY AUTOINCREMENT,
                              name TEXT,
                              email TEXT,
                              phone TEXT)''')
          cursor.execute('INSERT INTO submissions (name, email, phone) VALUES (?, ?, ?)', (name, email, phone))
          conn.commit()
          conn.close()
  
          return 'Success'
  
      @app.route('/')
      def index():
          return render_template_string('''
              <form method="post" action="/submit">
                  <input type="text" name="name" placeholder="Name"><br>
                  <input type="email" name="email" placeholder="Email"><br>
                  <input type="text" name="phone" placeholder="Phone"><br>
                  <input type="submit" value="Submit">
              </form>
          ''')
  
      with app.test_client() as client:
          response = client.post('/submit', data={'name': 'Test', 'email': 'test@example.com', 'phone': '1234567890'})
          if response.data.decode('utf-8') == 'Success':
              return 'Success'
          else:
              return 'Failure'