Add coverage for GPURenderPassDescriptor.timestampWrites a la compute passes

[ErichDonGubler]

There is currently no direct render pass equivalent for most of the validation coverage for GPUComputePassDescriptor.timestampWrites. We should do the same validation for render passes as for compute passes, i.e., we need a render pass variant of webgpu:api,validation,encoding,beginComputePass:timestampWrites,*.