change signal categories

[sgratzl]

from cmu-delphi/www-covidcast#1049

add a new category: cases_testing

Cases and Testing: Cases, Antigen-TPR (Quidel), PCR-TPR (CAN)

also adapt code

delphi-epidata/src/server/endpoints/covidcast_utils/model.py

Lines 26 to 30 in f8cb065

	class SignalCategory(str, Enum):
	public = "public"
	early = "early"
	late = "late"
	other = "other"