fix: Restore missing file `HelpTest.java` on an accurate folder.

_Ref._:
- 9c62daa35d
- e739cd8f5d

test/testhelper/TestHelper.java

@@ -0,0 +1,23 @@
+package testhelper;
+
+import static net.sourceforge.plantuml.test.PlantUmlTestUtils.exportDiagram;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class TestHelper {
+
+	@Test
+	public void test_help_themes() throws Exception {
+
+		final String output = exportDiagram(
+				"@startuml",
+				"help themes",
+				"@enduml"
+		).asString();
+
+		assertThat(output)
+				.startsWith("                                \nHelp on themes")
+				.contains("bluegray", "hacker");
+	}
+}