utc2et test and kernel checking added

I've added a test for utc2et (my first :+1: ). I based it on the example given in the NAIF docs for the icy version of utc2et. There is a slight difference of c. 5 seconds between the output value given in the icy example and the value SpiceyPy gives for the conversion.

I've also added a check to getkernels so that the kernels are only downloaded if they don't exist in the cwd... saves them being downloaded every time the tests are run.

This is my first contribution to a github project other than my own and my first pull request, so if I have missed a step or left out some vital info. please let me know.

J.
I would say try to have at least 5 or 10 tests per PR, but feel free to have more, the issue being that I could for example start writing a test that you had written in a PR you were prepping with 50 test which would create conflicts, while not impossible to fix are easy to avoid with a PR every other day or so. This PR is good, and I will merge it now.