tests: move JSON escape test to test-hgweb-json.t The original tests (kanji and null) in test-hgweb-commands.t come from aff419e260f9 and 823a7d79ef82, but they check json escape filter by using JavaScript variable on /graph page, which is awkward, and I'm planning to remove commit description from this variable soon. Let's move the parts that check json template filter to a more appropriate file and use normal json-* templates.