enhance tests (mostly codegen)

* drop dependency on stdlib/alt-headers in the most tests
* assign to myEnvironment at most once in JetLiteFixture
* more tests do not compile runtime

compiler/tests/org/jetbrains/jet/codegen/TraitsTest.java

@@ -16,7 +16,16 @@
 
 package org.jetbrains.jet.codegen;
 
+import org.jetbrains.jet.lang.resolve.java.CompilerSpecialMode;
+
 public class TraitsTest  extends CodegenTestCase {
+
+    @Override
+    protected void setUp() throws Exception {
+        super.setUp();
+        createEnvironmentWithMockJdk(CompilerSpecialMode.JDK_HEADERS);
+    }
+
     @Override
     protected String getPrefix() {
         return "traits";