Expose random seed and make static test nullable

[mkorbel1]

Description & Motivation

This PR refactors some parts of the Test API.

• Static members related to Test.instance, including Test.random, are now nullable instead of late. This highlights the need for a Test to be constructed before using those variables, and avoids confusing late initialization errors.
• Expose the randomSeed for Tests so that it can be queried, printed, etc. to make it easier to reproduce issues.
• Make the Test.instance null out after a test has run, so that it doesn't stick around afterwards, which could cause some confusion.
• Adds Test.reset() to reset the Test.instance.

Related Issue(s)

Fix #24
Fix #23

Testing

Added tests around these changes

Backwards-compatibility

Is this a breaking change that will not be backwards-compatible? If yes, how so?

YES!

• Test.instance and Test.random are now nullable
• Test.instance is now a getter only.
• Test.instance is now null after a test is finished running.
• A Test will fail to construct (exception) if there's already an instance running.
• Should now use Test.reset() (which includes Simulator.reset())

Documentation

Does the change require any updates to documentation? If so, where? Are they included?

API docs are updated