Testing Kotlin Coroutines (Flow vs Publisher) - kotlin

Question

I have a reasonably simple bit of code - every second ping some handler with the current timestamp:
private val clock = Clock.systemDefaultZone()
private fun now() = clock.instant()
suspend fun ping(pinger: suspend (Instant) -> Unit) {
repeat(5) {
pinger(now())
if (it < 4) {
delay(1.seconds)
}
}
}
Which I want to expose as a reactive Producer:
fun publishPing() = publish { ping(::send) }
In practice, it works - but when testing:
#OptIn(ExperimentalCoroutinesApi::class)
#Test
fun `test publishPing`() = runTest {
var count = 0
publishPing().collect {
count++
assertTrue { EPOCH.isBefore(it) }
assertTrue { it.isBefore(now()) }
}
assertEquals(5, count) // Pass
assertEquals(4000, currentTime) // AssertionFailedError: Expected:4000; Actual:0
}
The virtual time doesn't get incremented, and he test takes 4+ seconds to run (eg the calls to delay() aren't being handled by the test dispatcher).
Doing the same with a flow all works as expected (and runs in milliseconds).
fun flowPing() = flow { ping(::emit) }
#OptIn(ExperimentalCoroutinesApi::class)
#Test
fun `test flowPing`() = runTest {
var count = 0
flowPing().collect {
count++
assertTrue { EPOCH.isBefore(it) }
assertTrue { it.isBefore(now()) }
}
assertEquals(5, count) // Pass
assertEquals(4000, currentTime) // Pass
}
I have a vague idea of what is [not] going on (the coroutines-test support isn't Hooked into the coroutines/reactive support?) - but what do I need to do to sort it?
FWIW the same behaviour happens with coroutines 1.5.x and runBlockingTest (code above from coroutines 1.6 and runTest)