Fix issue #450: buggy translation of constant clamp expressions

[DavidKnott]

- What I did

Fixed clamps in compile_lll.py
Added a fixture to make it easier to test LLL
Created LLL clamp tests
In response to issue #450, thanks @daejunpark

- How I did it

Corrected clamp constant checks in compile_lll.py

- How to verify it

Look at the tests I created in tests/compiler/test_clamps.py.

- Description for the changelog

None

- Cute Animal Picture

[jacqueswww]

Looks good to me, only thing one might have to consider is that assert_tx_failed is used in away that doesn't imply 'a transaction failed', instead 'the compiler failed' - just a minor note ;)

[jacqueswww]

👍 for the fixture :)

[DavidKnott]

@jacqueswww I created a fixture called assert_compile_failed which is just syntactic sugar for assert_tx_failed. Though I think this can be done better when we move a lot of setup_transaction_tests.py into conftest.py