Add integration tests for 'using declarations' with std::string

[calebfletcher]

Adding two integration tests for #1379. Each one produces the same underlying error but seemingly in different parts of autocxx.

Inlining the std::string in place of the string usages causes the tests to pass correctly, so this does appear to be something in autocxx or bindgen not correctly identifying std::string in order to replace it with CxxString.

• Tests pass
• Appropriate changes to README are included in PR

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[adetaylor]

Thanks for this - it'll make it much easier for me to take a look. I'm away for a few weeks at the moment so there will be a bit more of a delay.

[adetaylor]

Thanks, these tests no longer produce a problem on latest autocxx (0.27.1) - I've merged them for better test coverage.