[refactoring] model the precondition object

[ngarbezza]

Problem

We have a duplication problem that is generated by the repeated validations/preconditions we need to run on all the user input (suites, tests, assertions) before running a test suite or even considering it a valid one.

Proposed solution

Introduce the precondition object that is responsible for doing something like:

if(conditionIsNotMet) {
  throw new PreconditionError(errorMessage)
}

condition and error message must be provided by the user