Do not initialize non-mocked Dispatchers.Main unnecessarily

Before this change, the following could happen on the JVM:
* `kotlinx.coroutines.test` accesses `Dispatchers.Main` before
  `setMain` is called.
* `Dispatchers.Main` attempts to initialize *some other* Main
  dispatcher in addition to the `kotlinx-coroutines-test` Main
  dispatcher, if there is one.
* If the `kotlinx-coroutines-android` artifact is present + its
  R8 rules are used to minify the tests, it's illegal to
  attempt to fail to create a `Main` dispatcher.
  `SUPPORT_MISSING = false` ensures that attempting to create a
  Main dispatcher fails immediately, in the call frame that
  attempts the creation, not waiting for `Dispatchers.Main` to
  be meaningfully used.

In total, `kotlinx-coroutines-test` + `kotlinx-coroutines-android`
+ R8 minification of tests leads to some patterns of
`kotlinx-coroutines-test` usage to crash.

Fixes #4297

kotlinx-coroutines-test/common/src/internal/TestMainDispatcher.kt

@@ -9,31 +9,36 @@ import kotlin.coroutines.*
  * The testable main dispatcher used by kotlinx-coroutines-test.
  * It is a [MainCoroutineDispatcher] that delegates all actions to a settable delegate.
  */
-internal class TestMainDispatcher(delegate: CoroutineDispatcher):
+internal class TestMainDispatcher(createInnerMain: () -> CoroutineDispatcher):
     MainCoroutineDispatcher(),
     Delay
 {
-    private val mainDispatcher = delegate
-    private var delegate = NonConcurrentlyModifiable(mainDispatcher, "Dispatchers.Main")
+    internal constructor(delegate: CoroutineDispatcher): this({ delegate })
+
+    private val mainDispatcher by lazy(createInnerMain)
+    private var delegate = NonConcurrentlyModifiable<CoroutineDispatcher?>(null, "Dispatchers.Main")
 
     private val delay
         get() = delegate.value as? Delay ?: defaultDelay
 
     override val immediate: MainCoroutineDispatcher
         get() = (delegate.value as? MainCoroutineDispatcher)?.immediate ?: this
 
-    override fun dispatch(context: CoroutineContext, block: Runnable) = delegate.value.dispatch(context, block)
+    override fun dispatch(context: CoroutineContext, block: Runnable) =
+        (delegate.value ?: mainDispatcher).dispatch(context, block)
 
-    override fun isDispatchNeeded(context: CoroutineContext): Boolean = delegate.value.isDispatchNeeded(context)
+    override fun isDispatchNeeded(context: CoroutineContext): Boolean =
+        (delegate.value ?: mainDispatcher).isDispatchNeeded(context)
 
-    override fun dispatchYield(context: CoroutineContext, block: Runnable) = delegate.value.dispatchYield(context, block)
+    override fun dispatchYield(context: CoroutineContext, block: Runnable) =
+        (delegate.value ?: mainDispatcher).dispatchYield(context, block)
 
     fun setDispatcher(dispatcher: CoroutineDispatcher) {
         delegate.value = dispatcher
     }
 
     fun resetDispatcher() {
-        delegate.value = mainDispatcher
+        delegate.value = null
     }
 
     override fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>) =

kotlinx-coroutines-test/jvm/src/internal/TestMainDispatcherJvm.kt

@@ -8,8 +8,7 @@ internal class TestMainDispatcherFactory : MainDispatcherFactory {
     override fun createDispatcher(allFactories: List<MainDispatcherFactory>): MainCoroutineDispatcher {
         val otherFactories = allFactories.filter { it !== this }
         val secondBestFactory = otherFactories.maxByOrNull { it.loadPriority } ?: MissingMainCoroutineDispatcherFactory
-        val dispatcher = secondBestFactory.tryCreateDispatcher(otherFactories)
-        return TestMainDispatcher(dispatcher)
+        return TestMainDispatcher({ secondBestFactory.tryCreateDispatcher(otherFactories) })
     }
 
     /**
@@ -24,4 +23,4 @@ internal actual fun Dispatchers.getTestMainDispatcher(): TestMainDispatcher {
     val mainDispatcher = Main
     require(mainDispatcher is TestMainDispatcher) { "TestMainDispatcher is not set as main dispatcher, have $mainDispatcher instead." }
     return mainDispatcher
-}
+}