add some new string(char, int) benchmarks (#2132)

Author: adamsitnik

src/benchmarks/micro/libraries/System.Runtime/Perf.String.cs

@@ -20,6 +20,12 @@ public static IEnumerable<object> TestStringSizes()
             yield return new StringArguments(1000);
         }
 
+        [Benchmark]
+        [Arguments(1)]
+        [Arguments(10)]
+        [Arguments(100)]
+        public string CtorCharCount(int size) => new string('a', size);
+
         [Benchmark]
         [ArgumentsSource(nameof(TestStringSizes))]
         public char[] GetChars(StringArguments size) // the argument is called "size" to keep the old benchmark ID, do NOT rename it