Make testDelayChannelBackpressure2 not fail

This test could in theory already fail on the second
`checkNotEmpty`: after the first `checkNotEmpty` has passed,
first, the ticker channel awakens to produce a new element, and
then the main thread awakens to check if it's there. However, if
the ticker channel is sufficiently slowed down, it may not produce
the element in time for the main thread to find it.

After introducing the change that allows the worker thread in
`DefaultExecutor` to shut down, the initial delay of 2500 ms is
sufficient for the worker to shut down (which, by default, happens
after 1000 ms of inactivity), and then the aforementioned race
condition worsens: additional time is required to launch a new
worker thread and it's much easier to miss the deadline.

Now, the delays are much shorter, meaning that the worker thread is
never inactive long enough to shut down.

Author: dkhalanskyjb

kotlinx-coroutines-core/jvm/test/channels/TickerChannelTest.kt

@@ -47,17 +47,17 @@ class TickerChannelTest : TestBase() {
     @Test
     fun testDelayChannelBackpressure2() = withVirtualTimeSource {
         runTest {
-            val delayChannel = ticker(delayMillis = 1000, initialDelayMillis = 0)
+            val delayChannel = ticker(delayMillis = 200, initialDelayMillis = 0)
             delayChannel.checkNotEmpty()
             delayChannel.checkEmpty()
 
-            delay(2500)
+            delay(500)
             delayChannel.checkNotEmpty()
-            delay(510)
+            delay(110)
             delayChannel.checkNotEmpty()
-            delay(510)
+            delay(110)
             delayChannel.checkEmpty()
-            delay(510)
+            delay(110)
             delayChannel.checkNotEmpty()
             delayChannel.cancel()
         }