j2k: flatten test cases and testData directory structure

Move j2k/test/tests -> j2k/tests, j2k/test/testData -> j2k/testData

j2k/testData/fileOrElement/nullability/MethodResultInitializesNullableField.java

@@ -0,0 +1,8 @@
+//file
+import org.jetbrains.annotations.Nullable;
+
+class C {
+    @Nullable private final String string = getString();
+
+    static String getString() { return x(); }
+}