Better testing of SyntaxNode -> Expr conversion

[c42f]

As exposed by #113, we had some unnecessary regressions due to the Expr conversion code being under-tested.

We need to make sure each branch in src/expr.jl is covered with the tests in test/expr.jl

[update] Some tests are now hosted in test/parser.jl, but it'd be better to move them into test/expr.jl and decouple them from the other tests, I think.