text: Title: Add `robot.running.TestSuite.from_string` method
  Body: It's already possible to do this:
  ```python
  >>> from robot.api import get_model, TestSuite
  >>> suite = TestSuite.from_model(get_model('''
  ... *** Test Cases ***
  ... Example
  ...     Log    Hello, world!
  ... '''))
  ```
  Using `get_model` here is somewhat unnecessary and this would be more convenient:
  
  ```python
  >>> from robot.api import TestSuite
  >>> suite = TestSuite.from_string('''
  ... *** Test Cases ***
  ... Example
  ...     Log    Hello, world!
  ... '''))
  ```
  The `from_string` method would need to accept configuration options that `get_model` accepts. Instead of accepting individual options, and needing to update them if `get_model` is changed, it's probably better to accept `**config` that is passed to `get_model`. The existing `TestSuite.from_file_system` works that way as well.