Utils: Improve tests for small string

It fixes a capacity bug.

Change-Id: I3545eb32d8b23514da286c2df77aebc197df8252
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/smallstring.h

@@ -39,6 +39,7 @@ public:
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     using size_type = std::size_t;
+    static constexpr size_type Capacity = Size;
 
     static_assert(Size < 64 ? sizeof(Internal::StringDataLayout<Size>) == Size + 1
                             : sizeof(Internal::StringDataLayout<Size>) == Size + 16,

src/libs/utils/smallstringlayout.h

@@ -264,7 +264,7 @@ struct alignas(16) StringDataLayout<MaximumShortStringDataAreaSize,
 
     StringDataLayout(const char *string, size_type size, size_type capacity) noexcept
         : size_{static_cast<int>(size)}
-        , capacity_{static_cast<int>(capacity)}
+        , capacity_{std::max<int>(capacity, MaximumShortStringDataAreaSize)}
     {
         if (Q_LIKELY(capacity <= shortStringCapacity())) {
             std::char_traits<char>::copy(buffer, string, size);