test: create JunitUtils to help test with Junit

- [x]  create `JunitUtils`to help test with Junit
  - create `StringJsonConverter` class to use with `@ConvertWith`

test/net/sourceforge/plantuml/test/JunitUtils.java

@@ -0,0 +1,26 @@
+package net.sourceforge.plantuml.test;
+
+import org.junit.jupiter.params.converter.ArgumentConversionException;
+import org.junit.jupiter.params.converter.SimpleArgumentConverter;
+
+import net.sourceforge.plantuml.json.Json;
+import net.sourceforge.plantuml.json.JsonValue;
+
+/**
+ * Class to help test with `Junit`.
+ */
+public class JunitUtils {
+	/**
+	 * `StringJsonConverter` class to use with `@ConvertWith`.
+	 */
+	public static class StringJsonConverter extends SimpleArgumentConverter {
+		@Override
+		protected Object convert(Object source, Class<?> targetType) throws ArgumentConversionException {
+			if (source instanceof String) 
+				return (JsonValue) Json.parse((String) source);
+			else
+				throw new IllegalArgumentException("Conversion from " + source.getClass() + " to "
+									    			+ targetType + " not supported.");
+		}
+	}
+}