BOOST_<level>_NO_THROW with return value

I'm using BOOST_REQUIRE_NO_THROW() as follows:

```cpp
#define BOOST_TEST_MAIN
#include

struct nc {
nc(int v): v { v } {}
nc() = delete;
nc(nc const&) = delete;
nc(nc&&) = default;
int v;
};

nc success() {
return nc(1);
}

nc fail() {
throw(42);
return nc(2);
}

BOOST_AUTO_TEST_CASE(test_case1) {
BOOST_REQUIRE_NO_THROW(
auto v1 = success();
std::cout << v1.v << std::endl;
auto v2 = fail();
std::cout << v2.v << std::endl;
);
}
```

It works well. But output is not easy to understand.

Here is runnable demo:
https://wandbox.org/permlink/KnsKJ9xq9htug3iG

The output is:

```
Running 1 test case...
1
prog.cc(27): fatal error: in "test_case1": unexpected exception thrown by auto v1 = success(); std::cout << v1.v << std::endl; auto v2 = fail(); std::cout << v2.v << std::endl;

*** 1 failure is detected in the test module "Master Test Suite"
```

All lines in `BOOST_REQUIRE_NO_THROW()` seems to regard as one line.

Unfortunately the following notation is not accepted.

```cpp
BOOST_REQUIRE_NO_THROW(nc v1 = success());
std::cout << v1.v << std::endl; // no v1 in the scope
```

```cpp
nc v1 = BOOST_REQUIRE_NO_THROW(success()); // Syntax error
std::cout << v1.v << std::endl;
```

Finally I found a workaround using optional.

```cpp
#define BOOST_TEST_MAIN
#include
#include

struct nc {
nc(int v): v { v } {}
nc() = delete;
nc(nc const&) = delete;
nc(nc&&) = default;
int v;
};

nc success() {
return nc(1);
}

nc fail() {
throw(42);
return nc(2);
}

BOOST_AUTO_TEST_CASE(test_case1) {
std::optional v1;
BOOST_REQUIRE_NO_THROW(v1.emplace(success()));
std::cout << v1->v << std::endl;

std::optional v2;
BOOST_REQUIRE_NO_THROW(v2.emplace(fail()));
std::cout << v2->v << std::endl;
}
```

It works well and the error is reported accurate line.

Here is runnable demo:
https://wandbox.org/permlink/gEYLIxgofWub2G0j

The output is:

```
Running 1 test case...
1
prog.cc(28): fatal error: in "test_case1": unexpected exception thrown by v2.emplace(fail())

*** 1 failure is detected in the test module "Master Test Suite"
```

I think that the optional workaround is acceptable.
But if there is better way, I'd like to know.