test: add Fibonacci matrix exponentiation tests

src/test/java/com/thealgorithms/matrix/matrixexponentiation/FibonacciTest.java

@@ -0,0 +1,34 @@
+package com.thealgorithms.matrix.matrixexponentiation;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import java.math.BigDecimal;
+import org.junit.jupiter.api.Test;
+class FibonacciTest {
+    private static BigDecimal[][] matrix(String a, String b, String c, String d) {
+        return new BigDecimal[][] {{new BigDecimal(a), new BigDecimal(b)}, {new BigDecimal(c), new BigDecimal(d)}};
+    }
+    @Test
+    void testFibZeroReturnsIdentityMatrix() {
+        assertArrayEquals(matrix("1", "0", "0", "1"), Fibonacci.fib(0));
+    }
+    @Test
+    void testFibOne() {
+        assertArrayEquals(matrix("1", "1", "1", "0"), Fibonacci.fib(1));
+    }
+    @Test
+    void testFibEvenInput() {
+        assertArrayEquals(matrix("2", "1", "1", "1"), Fibonacci.fib(2));
+    }
+    @Test
+    void testFibOddInput() {
+        assertArrayEquals(matrix("3", "2", "2", "1"), Fibonacci.fib(3));
+    }
+    @Test
+    void testFibTen() {
+        assertArrayEquals(matrix("89", "55", "55", "34"), Fibonacci.fib(10));
+    }
+    @Test
+    void testFibLargeInput() {
+        assertArrayEquals(matrix("20365011074", "12586269025", "12586269025", "7778742049"), Fibonacci.fib(50));
+    }
+}