[lldb][test] Merge MSVC STL std::(u8)string tests into generic directory (#147525)

Now that most STL formatter tests have been moved to `generic`. Do the
same for the MSVC tests (which are currently just for `std::string`).
The `std::string` test was mostly the same (MSVC just had 1 additional
check, which I moved over).

We also only tested `u8string` with MSVC. So i moved those into
`generic` as-is. I kept it separate from the existing std::string tests
since it requires c++20.

The tests are currently failing for libc++ and libstdc++ because MSVC
had a test case which checked that:
```
  std::string overwritten_zero("abc");
  const_cast<char *>(overwritten_zero.data())[3] = 'd';
```
prints as `"abc"`. But libc++ and libstdc++ print it as `"abcd"` (which
seems like the more correct thing to do?)

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