Wemake-python-styleguide: Add `NamedExpr` template to name testing

[sobolevn]

We should test that NamedExpr follows all the same rules that other ways to assign values.

We should modify tests/test_visitors/test_ast/test_naming/conftest.py to add a new template there. I think, that it should be enough. All other tests will work from there.

Here's what this template should look like:

if {0} := other_call(): 
    ...

We should only add this template for python3.8+

Related #1137

[jigi-33]

Hello, can I take this issue?

[sobolevn]

Sure! Thank you!

[sobolevn]

And one more case: lambda {0}, /: ...

[Hiyorimi]

On it