Fix for clang static analyzer issue CWE-119

Switch to `strlcpy` since it is safer to use and remove `strcpy` since it is insecure (although I don't know why unit test code would try to manipulate the buffer, but better safe than sorry).
Like this?
```objective-c
strlcpy(buffer, cStringPtr, 100);
strlcpy(buffer + 100, "...", sizeof(buffer) - 100);
```