As you probably noticed C & C++ are different languages.

Is it possible to make it compile when included into C++ code without making the struct type declaration untested?

Yes it is:

#include <stdio.h>

struct foo {
    struct bar {
        int baz;
    } bar;
};

int main() {
    struct foo::bar a;  // Use foo::bar to refer to the nested struct
    a.baz = 2;
    printf("%d", a.baz);
    return 0;
}