JSpecify: rework tests – replace kotlin use sites to java ones with future j2k conversion

compiler/testData/foreignAnnotationsJava8/tests/jspecify/java/Simple.java

@@ -0,0 +1,33 @@
+import org.jspecify.annotations.*;
+
+@DefaultNonNull
+public class Simple {
+    @Nullable public Derived field = null;
+
+    @Nullable
+    public Derived foo(Derived x, @NullnessUnspecified Base y) {
+        return null;
+    }
+
+    public Derived bar() {
+        return null;
+    }
+}
+
+class Base {}
+class Derived extends Base {
+    void foo() {}
+}
+
+@DefaultNonNull
+class Use {
+    static public void main(Simple a, Derived x) {
+        a.foo(x, null).foo();
+        // jspecify_nullness_mismatch
+        a.foo(null, x).foo();
+
+        a.bar().foo();
+
+        a.field.foo();
+    }
+}