Feature: Validate FillPrice and FillQuantity in class BrokerageTest

[Romazes]

Description

The problem, when a developer is implementing a new plugin for brokerage, is that he can miss returning several important parameters which give wrong calculations of holding quantity.
This PR provides protection and validates that 2 properties are greater than 0.

Related Issue

N/a

Motivation and Context

Eliminate the existing bug in Futures to facilitate the development of new Lean plugins.

Requires Documentation Change

N/A

How Has This Been Tested?

Run some inner trading tests of any Lean Brokerages plugin.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance. Please add associated performance test and results)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>

[Martin-Molinero]

Thank you 👍