test_expr_fns safe against no-caching

In general the `basic` contruction functions can't be used with
Expr arithmetic operators.  The tree needs to be constructed explicitly

This commit replaces x + 1 with Basic.__new__(Add, x, 1)
and removes the equality test prior to reconstruction
Merge pull request #6 from mrocklin/cacheless

test_expr_fns safe against no-caching