test: add tests for /advisory atom and json output
test: add tests for /advisory atom and json output

Create four simple tests which check an empty feed/json endpoint and one
with a published advisory.

anthraxx/arch-security-tracker,jelly/arch-security-tracker,archlinux/arch-security-tracker,anthraxx/arch-security-tracker,jelly/arch-security-tracker,archlinux/arch-security-tracker,anthraxx/arch-security-tracker