ultra tiny nit: avoid using globals for testing

[bennetyee]

communicating through a global for testing is a little ugly. consider something like:

func isNeutralVartime(p *ge25519.Ge25519) bool {
  return isNeutralVartimeTestInterface(p, nil)
}
func isNeutralVartimeTestInterface(p *ge25519.Ge25519, saveBatchY []byte) bool {
  if saveBatchY != nil {
    curve25519.Contract(saveBatchY, p.Y())
  }
  ...
}

which should, if go compilers ever get that level of optimization, allow generation of an if saveBatchY-free version of the code for callsites where the isNeutralVartime is used. also, if golang testing ever changes to allow running tests concurrently (seems unlikely), this eliminates the potential debt.

ed25519/batch_verify.go

Line 257 in cbed068

if testBatchSaveY {