Refactor codegen tests

- initialize environment only once in setUp()
- add comments on why some tests are disabled
- modify and rename some tests
- re-enable now working tests
- extract some tests into files with box()
- remove useless 'throws' declarations and commented code

compiler/testData/codegen/extensionFunctions/intCountDownLatchExtension.kt

@@ -0,0 +1,21 @@
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.locks.ReentrantLock;
+
+fun <T> Int.latch(op:  CountDownLatch.() -> T) : T {
+    val cdl = CountDownLatch(this)
+    val res = cdl.op()
+    cdl.await()
+    return res
+}
+
+fun id(op : ()->Unit) = op()
+
+fun box() : String {
+    1.latch{
+        id {
+            countDown()
+        }
+    }
+    return "OK"
+}