rpctest: Fix typo
rpctest: Fix typo

Use os.Getpid() to get the process ID, not os.Getppid(), which returns
the parent process ID.  This resulted in multiple calls to
generateListeningAddresses() getting the same listening ports.

wallclockbuilder/btcd,jcmincke/dcrd,ltcsuite/ltcd,stevenroose/btcd,davecgh/btcd,jrick/btcd,jrick/dcrd,Stash-Crypto/btcd,jcmincke/dcrd,Roasbeef/btcd,decred/dcrd,aakselrod/btcd,davecgh/btcd,Stash-Crypto/btcd,jrick/dcrd,wallclockbuilder/btcd,adiabat/btcd,tb00/btcd,cqswxj2013/btcd,stevenroose/btcd,ppcsuite/ppcd,btcsuite/btcd,jongillham/btcd,cqswxj2013/btcd,tb00/btcd,dajohi/btcd,dajohi/btcd,ppcsuite/ppcd,aakselrod/btcd,ltcsuite/ltcd,jrick/btcd,jcmincke/dcrd,jongillham/btcd,jrick/btcd,dajohi/btcd,btcsuite/btcd,adiabat/btcd,Roasbeef/btcd,BitGo/prova,decred/dcrd,BitGo/prova