TST: add temporary elision testcases

A very significant optimization for numpy would be to avoid the
temporaries in these types of expressions:

    res = a + b + c

transforming it into:

    res = a + b
    res += c

An approach to do that is to check the reference count of the PyNumber
slot arguments, temporary expressions coming from python have reference
count 1 and can be converted to inplace operations.
Unfortunately C-extensions can skip increasing reference counts for
operations breaking the assumption that an inplace operation can be
performed, e.g. Cython does this type of optimizations.
To ensure to not accidentally break such extensions in future test
that refcount == 1 operands from extensions are not elided.
Merge pull request #4856 from juliantaylor/elide-testcases

TST: add temporary elision testcases