wstring -> string, cherry-picked.

```cpp
std::wstring ToString(Dimensionless const&)
```
will be removed in the process of adding matchers, this fixes the old tests that relied on it.
``Assert::AreEqual`` is used for types that have a ``std::wstring ToString(T)`` defined by Microsoft, whereas ``AssertEqual`` is used for types that have a ``std::string ToString``.