In the functional tests, we often compare dicts with assert_equal, but the
output makes it very hard to tell exactly which entry in the dicts don't
match when there are a lot of entries and only minor differences. Change
the output to make it clearer.

Moves FormatScriptFlags logic into GetScriptFlagNames which returns a
vector of strings. For completeness, also has GetScriptFlagNames report
on any bits that do not match a known script flag.