[conformal/btcec] 4ca0da: Add test for private key serialization.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcec
  Commit: 4ca0daacc139d145eab60124a16192265da27acb
      
https://github.com/conformal/btcec/commit/4ca0daacc139d145eab60124a16192265da27acb
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-08-10 (Sun, 10 Aug 2014)

  Changed paths:
    A privkey_test.go
    M pubkey_test.go

  Log Message:
  -----------
  Add test for private key serialization.

Also move private key tests to their own file while here.