Add sort by config and tests for Reddit integration

[ljmerza]

Breaking Change:

Description:

Adds sort by config to Reddit sensor and tests.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8952

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: reddit
    username: !secret reddit_username
    password: !secret reddit_password
    client_id: !secret reddit_client_id
    client_secret: !secret reddit_client_secret
    sort_by: new
    subreddits:
      - news
      - worldnews

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.