[SR-14424] Add 'moveInitialize' test case for small string 'init(unsafeUninitializedCapacity:initializingUTF8With:)' (swiftlang#36742)

* [SR-14424] Add 'moveInitialize' test case for small string 'init(unsafeUninitializedCapacity:initializingUTF8With:)'

...and make an adjustment to an existing test case.

* [stdlib][test] Update test cases for small string unsafeUninitializedCapacity based on reviewer feedback.

* [fixup]

Author: xwu

test/stdlib/StringCreate.swift

@@ -94,13 +94,34 @@ if #available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *) {
     }
     expectEqual(str1, str3)
 
-    // Write into uninitialized space
+    // Write into uninitialized space.
     let str4 = String(unsafeUninitializedCapacity: 3) {
       $0[0] = UInt8(ascii: "4")
       $0[1] = UInt8(ascii: "2")
       $0[2] = UInt8(ascii: "X")
+      // Deinitialize memory used for scratch space before returning.
+      ($0.baseAddress! + 2).deinitialize(count: 1)
       return 2
     }
     expectEqual(str1, str4)
+
+    let str5 = String(unsafeUninitializedCapacity: 3) {
+      $0[1] = UInt8(ascii: "4")
+      $0[2] = UInt8(ascii: "2")
+      // Move the initialized UTF-8 code units to the start of the buffer,
+      // leaving the non-overlapping source memory uninitialized.
+      $0.baseAddress!.moveInitialize(from: $0.baseAddress! + 1, count: 2)
+      return 2
+    }
+    expectEqual(str1, str5)
+
+    // Ensure reasonable behavior despite a deliberate API contract violation.
+    let str6 = String(unsafeUninitializedCapacity: 3) {
+      $0[0] = UInt8(ascii: "4")
+      $0[1] = UInt8(ascii: "2")
+      $0[2] = UInt8(ascii: "X")
+      return 2
+    }
+    expectEqual(str1, str6)
   }
 }