unit_test.hpp successfully compiles invalid code

I'm resubmitting this bug because the [last one](https://github.com/boostorg/test/issues/312) was closed and I'm afraid the nature of the issue was not clear.

Consider this invalid code:
```
BOOST_TEST(1 = somestring.size());
```

Assignment to a literal should be a compilation error, however the code above compiles successfully when using unit_test.hpp.

See: https://gcc.godbolt.org/z/MT8xrdesh