TFLite XNNPack Delegate: use kernel size of 5 for correspondingly named tests
The [documentation](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/delegates/xnnpack/README.md#sparse-inference) explicitly states that 5x5 kernels are supported and the test heads suggest, that kernel sizes of 5 should be tested, but actually, kernel sizes of 3 are tested. This PR changes the kernel sizes tested to match the test heads. 

Disclaimer: I haven't run the tests yet, I only saw this on github and created this PR.