Add coverage for validation of >=1 target in render passes

[ErichDonGubler]

Following up from https://github.com/gpuweb/cts/issues/3754, we should add coverage that checks validation for at least one render target. See also section 10.3.1. (Render Pipeline Creation), which states in the subsection titled validating GPURenderPipelineDescriptor(descriptor, layout, device):

There must exist at least one attachment, either:

• A non-null value in descriptor.fragment.targets, or

• A descriptor.depthStencil.