Add Default Values for HarnessFunctionName and HarnessModulePath in AppVeyor\Invoke-AppveyorTestScriptTask cmdlet

[PlagueHO]

This will simplify the AppVeyor.yml in resources that use a Test Harness if they choose to use the standard names. It will be up to each resource maintainer if they use the defaults or override them. xNetworking, xStorage etc will be changed to use standard names. I think Johan Ljunggren (@johlju) is also planning to make xSQLServer use the standard names. Unsure about SharePointDsc.

The values will default to:

        $HarnessModulePath = 'Tests\TestHarness.psm1',
        $HarnessFunctionName = 'Invoke-TestHarness'

See dsccommunity/NetworkingDsc#186 (comment)

Will submit a PR now.