Use the TestCoroutineDispatcher from Dispatchers.Main by default

[dkhalanskyjb]

This change eliminates the common pattern that is currently needed to be able to use Dispatchers.Main in code under test:

val dispatcher = TestCoroutineDispatcher()
val scope = TestCoroutineScope(dispatcher)

@BeforeTest
fun setMain() {
  Dispatchers.setMain(dispatcher)
}

@AfterTest
fun resetMain() {
  Dispatcher.resetMain()
  scope.cleanupTestCoroutines()
}

@Test
fun testFoo() = scope.runBlockingTest { ... }

Now this becomes just

@BeforeTest
fun setMain() {
  Dispatchers.setMain(TestCoroutineDispatcher())
}

@AfterTest
fun resetMain() {
  Dispatcher.resetMain()
}

@Test
fun testFoo() = runTest { ... }

This is done by querying Dispatchers.Main for its TestCoroutineScheduler.

[qwwdfsad]

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