Ugh this is a bit tricky since time is implemented as dedicated part of the grammar in order to support timing fish functions and blocks.
When adding time as generic builtin, the tests fail because a=b time is supposed to call the external command time, which it doesn't if it is a builtin. This behavior might not be too important, because one can use the more explicit a=b command time instead. So we could break it (changing the test), or add some workarounds like we do for and/or.
Also time -h and time --help are not supported currently. Similar story here.