[conformal/btcec] cc7128: Add tests for pad function.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcec
  Commit: cc712827daf8fbec04d6bc215ded5e8e8f2d6cb0
      
https://github.com/conformal/btcec/commit/cc712827daf8fbec04d6bc215ded5e8e8f2d6cb0
  Author: Dave Collins <da...@conformal.com>
  Date:   2013-12-28 (Sat, 28 Dec 2013)

  Changed paths:
    M internal_test.go
    M pubkey_test.go
    M test_coverage.txt

  Log Message:
  -----------
  Add tests for pad function.

This brings the overall test coverage up to 99.88%.