DscResources/CONTRIBUTING.md: Test Templates-link is stale

[p0shkar]

In the Write Tests-section of DscResources/CONTRIBUTING.md the Test Templates-link is not working anymore.

We highly encourage you to use our test templates when creating tests for DSC resources.

[johlju]

Suggest looking here https://github.com/PowerShell/DscResource.Template/tree/master/Tests.

Those templates should be moved over to the Sampler repo (see issue gaelcolas/Sampler#108). Also this the guideline here should be update https://dsccommunity.org/guidelines/testing-guidelines/. But that guide assumes the new pipeline is used (the one in the Plaster template in the Sampler project), see https://dsccommunity.org/blog/convert-a-module-for-continuous-delivery/