refactor: DecimalToOctal (#5332)

src/test/java/com/thealgorithms/conversions/DecimalToOctalTest.java

@@ -0,0 +1,21 @@
+package com.thealgorithms.conversions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class DecimalToOctalTest {
+    @ParameterizedTest
+    @CsvSource({"0, 0", "7, 7", "8, 10", "10, 12", "64, 100", "83, 123", "7026, 15562"})
+    void testConvertToOctal(int decimal, int expectedOctal) {
+        assertEquals(expectedOctal, DecimalToOctal.convertToOctal(decimal));
+    }
+
+    @Test
+    void testConvertToOctalNegativeNumber() {
+        assertThrows(IllegalArgumentException.class, () -> DecimalToOctal.convertToOctal(-10));
+    }
+}