Fix Java6BuiltInsWithJDKMembersTest

After update of the project to JDK 8, the default JDK at java.home is
now JDK 8. However, this test relied on the fact that the default is JDK
6. Pass the path to the JDK 6 explicitly

compiler/tests-common/org/jetbrains/kotlin/test/KotlinTestUtils.java

@@ -468,6 +468,9 @@ public class KotlinTestUtils {
         else if (jdkKind == TestJdkKind.ANDROID_API) {
             JvmContentRootsKt.addJvmClasspathRoot(configuration, findAndroidApiJar());
         }
+        else if (jdkKind == TestJdkKind.FULL_JDK_6) {
+            JvmContentRootsKt.addJvmClasspathRoots(configuration, PathUtil.getJdkClassesRoots(System.getenv("JDK_16")));
+        }
         else {
             JvmContentRootsKt.addJvmClasspathRoots(configuration, PathUtil.getJdkClassesRoots());
         }

compiler/tests-common/org/jetbrains/kotlin/test/TestJdkKind.java

@@ -21,6 +21,9 @@ public enum TestJdkKind {
     // Differs from common mock JDK only by one additional 'nonExistingMethod' in Collection and constructor from Double in Throwable
     // It's needed to test the way we load additional built-ins members that neither in black nor white lists
     MODIFIED_MOCK_JDK,
+    // JDK found at $JDK_16
+    FULL_JDK_6,
+    // JDK found at java.home
     FULL_JDK,
     ANDROID_API,
 }