tests: Add `eg.makeUser` to make fresh example users.
tests: Add `eg.makeUser` to make fresh example users.

Also generate various details of even the two pre-made users
("self" and "other") randomly, to prevent hardcoding them opaquely
in test cases.

(General point: randomness in tests is bad if it makes a test
occasionally fail, but fine if the test almost always fails.
I'm hoping this will mainly be the latter.)

The implementation of `randString` is inspired by what Redux does
for private action types like `ActionTypes.INIT`, with the same
purpose of preventing people from just hardcoding the strings in
their application code.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile