feat: `Random` function: add test and private declaration

Add:
- `RandomFunctionTest` test file
- private declaration for random lib.

Ref.:
- #1667

Improve:
- dbaf8ac2cd
- #1668

src/net/sourceforge/plantuml/tim/stdlib/RandomFunction.java

@@ -57,7 +57,7 @@ public class RandomFunction extends SimpleReturnFunction {
 		return nbArg == 0 || nbArg == 1 || nbArg == 2;
 	}
 
-	final Random random = new Random();
+	private final Random random = new Random();
 	
 	public TValue executeReturnFunction(TContext context, TMemory memory, LineLocation location, List<TValue> values,
 			Map<String, TValue> named) throws EaterException, EaterExceptionLocated {

test/net/sourceforge/plantuml/tim/stdlib/RandomFuntionTest.java

@@ -0,0 +1,50 @@
+package net.sourceforge.plantuml.tim.stdlib;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Arrays;
+import java.util.Collections;
+
+import org.junit.jupiter.api.DisplayNameGenerator.ReplaceUnderscores;
+import org.junit.jupiter.api.IndicativeSentencesGeneration;
+import org.junit.jupiter.api.RepeatedTest;
+
+import net.sourceforge.plantuml.tim.EaterException;
+import net.sourceforge.plantuml.tim.EaterExceptionLocated;
+import net.sourceforge.plantuml.tim.TFunction;
+import net.sourceforge.plantuml.tim.expression.TValue;
+
+/**
+ * Tests the builtin function.
+ */
+@IndicativeSentencesGeneration(separator = ": ", generator = ReplaceUnderscores.class)
+
+class RandomFunctionTest {
+	TFunction cut = new RandomFunction();
+	final String cutName = "Random";
+	final String repetitionLabel = "[{currentRepetition}/{totalRepetitions}] ";
+
+	@RepeatedTest(value = 10, name = repetitionLabel + cutName + "()")
+	void test_with_no_argument() throws EaterException, EaterExceptionLocated {
+		final TValue tValue = cut.executeReturnFunction(null, null, null, Collections.emptyList(), null);
+		assertThat(tValue.toInt()).isIn(0, 1);
+	}
+
+	@RepeatedTest(value = 10, name = repetitionLabel + cutName + "(7)")
+	void test_with_one_argument() throws EaterException, EaterExceptionLocated {
+		final TValue tValue = cut.executeReturnFunction(null, null, null, Arrays.asList(TValue.fromInt(7)), null);
+		assertThat(tValue.toInt()).isBetween(0, 7-1);
+	}
+
+	@RepeatedTest(value = 10, name = repetitionLabel + cutName + "(0, 7)")
+	void test_with_two_argument_first_zero() throws EaterException, EaterExceptionLocated {
+		final TValue tValue = cut.executeReturnFunction(null, null, null, Arrays.asList(TValue.fromInt(0), TValue.fromInt(7)), null);
+		assertThat(tValue.toInt()).isBetween(0, 7-1);
+	}
+
+	@RepeatedTest(value = 10, name = repetitionLabel + cutName + "(3, 7)")
+	void test_with_two_argument() throws EaterException, EaterExceptionLocated {
+		final TValue tValue = cut.executeReturnFunction(null, null, null, Arrays.asList(TValue.fromInt(3), TValue.fromInt(7)), null);
+		assertThat(tValue.toInt()).isBetween(3, 7-1);
+	}
+}