follow compare.sr.ht's rename to diff.sr.ht

The service renamed itself; what reaches this repository is the section name
in ecoretest's synthetic config.ini, which every service's switcher test is
built from, and the examples in the chrome docs and README that spelled the
old section. The fixture's custom services now read bench, cover, diff, dolt,
spec, tokens — alphabetical, which is the order BuildNav puts them in.

ecoretest: the shared instance config and crypto bootstrap for tests