tests/date: remove duplicate test cases
tests/date: remove duplicate test cases

glib 2.46.0 complains if you have two test cases with the same name,
but we were testing certain date formats multiple times.

jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,fanc999/libsoup,fanc999/libsoup,fanc999/libsoup,jwendell/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup