testlibrary: Make remote existence assertions more friendly
testlibrary: Make remote existence assertions more friendly

Currently the output produced when assert_remote_in_installation() or
assert_remote_not_in_installation() fails is not helpful: it doesn't
mention the remote name or where the assertion was called from. Fix
those problems by rewriting it as a #define.

(cherry picked from commit dd48e78652079556bebfe930caad2efa09656584)

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak