Add String.mapi tests for all new unrolling corner cases, and some general ones

Author: abelbraaksma

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs

@@ -79,11 +79,29 @@ type StringModule() =
 
     [<Test>]
     member this.MapI() =
-        let e1 = String.mapi (fun i c -> char(int c + i)) "foo"
-        Assert.AreEqual("fpq", e1)
+        let e1 = String.mapi (fun _ c -> c) "12345"
+        Assert.AreEqual("12345", e1)
 
-        let e2 = String.mapi (fun i c -> c) null 
-        Assert.AreEqual("", e2)
+        let e2 = String.mapi (fun _ c -> c + char 1) "1"
+        Assert.AreEqual("2", e2)
+
+        let e3 = String.mapi (fun _ c -> c + char 1) "AB"
+        Assert.AreEqual("BC", e3)
+
+        let e4 = String.mapi (fun i c -> char(int c + i)) "foo"
+        Assert.AreEqual("fpq", e4)
+
+        let e5 = String.mapi (fun _ c -> c) null 
+        Assert.AreEqual("", e5)
+
+        let e6 = String.mapi (fun _ c -> c) String.Empty 
+        Assert.AreEqual("", e6)
+
+        let e7 = String.mapi (fun _ _ -> failwith "should not fail") null 
+        Assert.AreEqual("", e7)
+
+        let e8 = String.mapi (fun i _ -> if i = 1 then failwith "should not fail" else char i) "X" 
+        Assert.AreEqual("\u0000", e8)
 
     [<Test>]
     member this.Filter() =