j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/NullableIntNoCrash.java

@@ -0,0 +1,8 @@
+import org.jetbrains.annotations.Nullable;
+
+//file
+class A {
+    int field = foo();
+
+    @Nullable int foo() { return 1; }
+}