test: LongestPalindromicSubstring (#5402)

* LongestPalindromicSubstring

* fix Rule:CollapsibleIfStatements

---------

Co-authored-by: alxkm <alx@alx.com>

src/test/java/com/thealgorithms/strings/LongestPalindromicSubstringTest.java

@@ -0,0 +1,21 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class LongestPalindromicSubstringTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCasesForLongestPalindrome")
+    void testLongestPalindrome(String input, String expected) {
+        assertEquals(expected, LongestPalindromicSubstring.longestPalindrome(input));
+    }
+
+    private static Stream<Arguments> provideTestCasesForLongestPalindrome() {
+        return Stream.of(Arguments.of("babad", "bab"), Arguments.of("cbbd", "bb"), Arguments.of("a", "a"), Arguments.of("", ""), Arguments.of("abc", "a"), Arguments.of(null, ""), Arguments.of("aaaaa", "aaaaa"));
+    }
+}