Improve test style

Get rid of Hamcrest, which is uncommon in this codebase, and
replace as many `assert` statements from other testing frameworks
as is reasonable with little automation by calls to
`kotlin.test.*`.
diff --git a/reactive/kotlinx-coroutines-rx2/test/FlowAsObservableTest.kt b/reactive/kotlinx-coroutines-rx2/test/FlowAsObservableTest.kt
index ab9e402..0908b34 100644
--- a/reactive/kotlinx-coroutines-rx2/test/FlowAsObservableTest.kt
+++ b/reactive/kotlinx-coroutines-rx2/test/FlowAsObservableTest.kt
@@ -6,9 +6,8 @@
 
 import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.*
-import org.hamcrest.core.*
-import org.junit.*
-import org.junit.Assert.*
+import org.junit.Test
+import kotlin.test.*
 
 class FlowAsObservableTest : TestBase() {
     @Test
@@ -39,7 +38,7 @@
         expect(2)
         observable.subscribe({ expectUnreached() }, { error ->
             expect(4)
-            assertThat(error, IsInstanceOf(RuntimeException::class.java))
+            assertTrue(error is RuntimeException)
             assertEquals("OK", error.message)
         })
         finish(5)