Fix String capacity growth tests for watchOS

watchOS devices can have different allocation characteristics
from other devices. This modifies the string capacity growth
tests to be more flexible about measuring the growth in
capacity, specifically when more is allocated than requested.

Author: natecook1000

validation-test/stdlib/String.swift

@@ -2380,7 +2380,8 @@ StringTests.test("String.CoW.reserveCapacity") {
   let preGrowCapacity = str.capacity
 
   // Growth shouldn't be linear
-  str.append(contentsOf: String(repeating: "z", count: 20))
+  let newElementCount = (preGrowCapacity - str.count) + 10
+  str.append(contentsOf: String(repeating: "z", count: newElementCount))
   expectGE(str.capacity, preGrowCapacity * 2)
 
   // Capacity can shrink when copying, but not below the count
@@ -2407,7 +2408,6 @@ StringTests.test("NSString.CoW.reserveCapacity") {
   var copy2 = str2
   copy2.append(contentsOf: String(repeating: "z", count: 10))
   expectGE(copy2.capacity, 30)
-  expectLT(copy2.capacity, 40)
 #endif
 }