Make TestSuite use namespace

[thoni56]

In #325 @TommyJC mentioned that TestSuite should also be namespaced "properly". Currently we have (in include/internal/suite_internal.h):

#ifdef __cplusplus
namespace cgreen {
    extern "C" {
#endif

void do_nothing(void);

TestSuite *create_named_test_suite_(const char *name, const char *filename, int line);
void add_test_(TestSuite *suite, const char *name, CgreenTest *test);
void add_tests_(TestSuite *suite, const char *names, ...);
void add_suite_(TestSuite *owner, const char *name, TestSuite *suite);

#ifdef __cplusplus
    }
}
#endif