In tests, strip all parentheses before pretty-printing

Seeing how many precedence checking the module does, I assume that a user of HSE can just pass a parentheses-free AST to the pretty-printer, and still expect properly-parenthesized code. However, HSE currently doesn't check this assumption.

After this patch, the test runner removes all parentheses between parsing and pretty-printing a test case.

This patch is blocked by a few bugs in HSE, which exemplify situations where parentheses should have been inserted, but HSE fails to do so.