Add New PyTest Based Test Runner

[mandli]

This PR introduces a new ClawpackTestRunner that utilizes PyTest more heavily to gain features and controls that were not possible or difficult to maintain/control just using the older unittest based framework.

Advantages include:

• Uses pytest more directly than the previous tests based on UnitTest
• Significant code simplification of both the test scaffoloding and individual tests
• Gives full control of the output (temporary) directory to the test via the commandline pytest --basetemp=
• Can specify which setrun is used by path and removes problems with collision of names
• Uses some pytest fixtures to provide other functionality meaning that they can also be expanded

The old ClawpackRegressionTest is still available for the time being. The other PRs in the tested repositories are:

• Switch to new pytest runner classic#96
• New pytest test runner amrclaw#310