refactor: Change HelpTest to CommandHelpThemeTest on a more accurate folder

To continue:
- #1655

Change `HelpTest.java` to `CommandHelpThemeTest.java`
Put on a more accurate folder:
- `net.sourceforge.plantuml.help`

test/net/sourceforge/plantuml/help/CommandHelpThemeTest.java

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