Select the asserter based on test framework presence in classpath

Simplify asserter lookup: enumerate all AsserterContributors when
the file class with lookupAsserter function is initialized.

It's assumed that AsserterContributor initialization should be fast.

#KT-21154 Fixed

libraries/kotlin.test/junit/src/main/kotlin/JUnitSupport.kt

@@ -3,21 +3,25 @@ package kotlin.test.junit
 import org.junit.*
 import kotlin.test.*
 
+/**
+ * Provides [JUnitAsserter] if `org.junit.Assert` is found in the classpath.
+ */
 class JUnitContributor : AsserterContributor {
     override fun contribute(): Asserter? {
-        for (stackFrame in currentStackTrace()) {
-            @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
-            val className = stackFrame.className as java.lang.String
+        return if (hasJUnitInClassPath) JUnitAsserter else null
+    }
 
-            if (className.startsWith("org.junit.") || className.startsWith("junit.")) {
-                return JUnitAsserter
-            }
-        }
-
-        return null
+    private val hasJUnitInClassPath = try {
+        Class.forName("org.junit.Assert")
+        true
+    } catch (_: ClassNotFoundException) {
+        false
     }
 }
 
+/**
+ * Implements `kotlin.test` assertions by delegating them to `org.junit.Assert` class.
+ */
 object JUnitAsserter : Asserter {
     override fun assertEquals(message: String?, expected: Any?, actual: Any?) {
         Assert.assertEquals(message, expected, actual)

libraries/kotlin.test/junit/src/test/kotlin/JUnitContributorTest.kt

@@ -1,27 +1,26 @@
 package kotlin.test.junit.tests
 
-import org.junit.*
+import org.junit.Assert
+import kotlin.test.*
 import java.util.concurrent.*
+import kotlin.test.junit.JUnitAsserter
 
 class JUnitContributorTest {
     @Test
     fun smokeTest() {
-        Assert.assertEquals("JUnitAsserter", asserter.`class`.simpleName)
+        assertSame(JUnitAsserter, kotlin.test.asserter)
+        Assert.assertEquals(JUnitAsserter::class.java.simpleName, kotlin.test.asserter.javaClass.simpleName)
     }
 
     @Test
-    fun `should fail to contribute if it was run outside of junit`() {
-        @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
-        val q = ArrayBlockingQueue<java.lang.Object>(1)
+    fun parallelThreadGetsTheSameAsserter() {
+        val q = ArrayBlockingQueue<Any>(1)
 
         Thread {
             q.put(asserter)
         }.start()
 
-        Assert.assertEquals("DefaultAsserter", q.take().`class`.simpleName)
+        assertSame(kotlin.test.asserter, q.take())
     }
 
-    @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
-    private val asserter: java.lang.Object
-        get() = Class.forName("kotlin.test.AssertionsKt").getMethod("getAsserter").invoke(null) as java.lang.Object
 }