nf-test for pilon

[thanhleviet]

replaced pytest with nf-test for pilon

[telatin]

I think pytest should be removed now. Quoting J F Yates:

Yes, if you add nf-test, remove pytest. The -m8grate-pytest` flag should do that for you if you do it via Nf-core tools

[SPPearce]

Please add a stub and test for the stub as mentioned by @itrujnara

[SPPearce]

@itrujnara , I've added the stub and a second test, if you want to take a look again.