added regression test and seperated out nested logic.

[TimothyStiles]

I'm not sure where the issue from #279 is coming from yet but I think it may have something to do with minimal primer length.

https://github.com/TimothyStiles/poly/blob/eabe734e6e074a4e912912962d05009b91b2236a/primers/pcr/pcr.go#L100

[gitpod-io[bot]]