Add test placeholders to prevent Gradle 9.0.0 from detecting missing tests

orx-gradient-descent/src/jvmTest/kotlin/TestPlaceholder.kt

@@ -0,0 +1,8 @@
+import kotlin.test.Test
+
+class TestPlaceholder {
+    @Test
+    fun testPlaceholder() {
+        assert(true)
+    }
+}