Leonlu2/test signal wildcard

[LeonLu2]

closes #950

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

1. add _insert_placeholder_set_four and test_signal_wildcard for signal=* testing
2. in covidcast.py add a signals="*" handler for parse_source_signal_pairs

[krivard]

👍 lgtm