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/jvm/src/main/kotlin/AsserterLookup.kt

@@ -1,48 +1,15 @@
 package kotlin.test
 
-import java.util.*
-import java.util.concurrent.atomic.*
-import java.util.concurrent.locks.*
-import kotlin.concurrent.withLock
+import java.util.ServiceLoader
 
-private val inited = AtomicBoolean()
-private val lock = ReentrantLock()
-private val contributors = ArrayList<AsserterContributor>()
-
-internal actual fun lookupAsserter(): Asserter = lookup()
+private val contributors = ServiceLoader.load(AsserterContributor::class.java).toList()
 
 private val defaultAsserter = DefaultAsserter()
 
-internal fun lookup(): Asserter {
-    initContributorsIfNeeded()
-
+internal actual fun lookupAsserter(): Asserter {
     for (contributor in contributors) {
         val asserter = contributor.contribute()
-        if (asserter != null) {
-            return asserter
-        }
+        if (asserter != null) return asserter
     }
-
     return defaultAsserter
-}
-
-private fun initContributors() {
-    contributors.clear()
-    val loader = ServiceLoader.load(AsserterContributor::class.java)
-
-    for (contributor in loader) {
-        if (contributor != null) {
-            contributors.add(contributor)
-        }
-    }
-}
-
-private fun initContributorsIfNeeded() {
-    if (!inited.get()) {
-        lock.withLock {
-            if (inited.compareAndSet(false, true)) {
-                initContributors()
-            }
-        }
-    }
-}
+}