Marbles cannot represent synchronous-upon-subscription notifications

[cartant]

Bug Report

This is more of a quirk - or a gotcha - than a bug, but it does mean that some tests in our test suite might be suspect and might not be testing quite what they are described as testing.

In this PR, it wasn't possible to reproduce a failing test for the bug because a marble source like this:

cold("(abcd|)")

emits multiple, synchronous notifications at frame zero, but - although the subscription occurs at frame zero - the notifications are not synchronous-upon-subscription.

At some stage, someone - me, I guess - is going to have to look through the tests to see whether or not tests that use sources like this are testing what they are intended to test - e.g. is this test testing what it's supposed to be testing?

Note that there are tests in the test suite that - like the test in the referenced PR - deliberately use non-marble sources to test this type of synchronous behaviour, so we just need to make sure that happens wherever that behaviour is tested.