Add tests

Author: EgorBo

src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs

@@ -694,7 +694,7 @@ public static bool Equals(string? a, string? b)
                     return EqualsUnrolled_0_to_8(a, b);
                 }
                 // Unroll using two Vector128s
-                if (b.Length <= 16 && Vector128.IsHardwareAccelerated)
+                else if (b.Length <= 16 && Vector128.IsHardwareAccelerated)
                 {
                     return EqualsUnrolled_9_to_16(a, b);
                 }