Don't raise if an assertion could not be found

The expected behaviour when you i.e call for a variable and it doesn't
exist is to get back `nil`. Let's not confuse anyone over that
Rename colliding variable `method` to `method_name`
Simplify `Test.order!`
Move `Test.test_order` to Runnable and rename to `Runnable.order`
Remove accessor to quell ruby warnings