Reusable BDD sections

One of pros the BDD autotesting is to facilitate reusability.
For example, you might have many tests needing the `GIVEN("My algo is in step A")` and you don't necessary want to push all those tests in the same scenario.
The idea would be to write the corresponding code for "My algo is in step A" only once and being able to reuse it anywhere.

The usage might looks like something like that:

```
REGISTER_GIVEN( "A vector with some items" ) {
...source code ...
}

GIVEN( "A vector with some items" ) {
// no code here -> the registered code should expand here

WHEN( "the size is reduced" ) {
v.resize( 0 );

THEN( "the size changes but not capacity" ) {
REQUIRE( v.size() == 0 );
REQUIRE( v.capacity() >= 5 );
}
}
}
```

Start by reviewing the existing BDD autotesting implementation for how GIVEN sections are defined and expanded. Determine how a named GIVEN could be registered once and reused in other scenarios, then verify that the example usage works without duplicating its setup code.

Written by the indexing model from the issue text.