reverse-string: Add a test with an even-sized word (#1519)
reverse-string: Add a test with an even-sized word (#1519)

Currently the tests have only odd sized strings. That allows an approach
to pass that does not work on even sized strings:

```cpp
std::string reverse_string::reverse_string(std::string str)
{
    if (str.size())
        for (auto a = str.begin(), b = str.end() - 1; a != b; ++a, --b)
            std::swap(*a, *b);
    return str;
}
```

The problem is `!=` instead of `<`.

Adding a test with an even sized string catches that invalid approach.
exercism/x-common,petertseng/x-common,ErikSchierboom/x-common,exercism/x-common,jmluy/x-common,ErikSchierboom/x-common,jmluy/x-common,petertseng/x-common,jmluy/x-common