gh-151284: Fix test_capi on UBSan

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Remove two checks relying on undefined behavior in test_fromwidechar() of test_capi.

Enable test_capi in GitHub Action "Reusable Sanitizer".

• Issue: Fix test_capi on UBSan #151284

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PyObject *
PyUnicode_FromWideChar(const wchar_t *u, Py_ssize_t size)
{
    ...

    if (find_maxchar_surrogates(u, u + size,
                                &maxchar, &num_surrogates) == -1)
        return NULL;

    ...
}

u + size with size=PY_SSIZE_T_MAX triggers an undefined behavior.

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@serhiy-storchaka: Would you mind to review this change?