Use `mktemp -d` for RBENV_TEST_DIR
Use `mktemp -d` for RBENV_TEST_DIR

This prevents the tests from removing an existing directory.

Fixes #561 (https://github.com/sstephenson/rbenv/issues/561)

OiNutter/nodenv,cehoffman/pyenv,sebhahn/pyenv,jmatbastos/rbenv,yyuu/pyenv,genome-vendor/pyenv,yyyc514/rbenv,sstephenson/rbenv,jmatbastos/rbenv,zmwangx/pyenv,jmatbastos/rbenv,jmatbastos/rbenv,jasonkarns/rbenv,pyenv/pyenv,BanzaiMan/pyenv,bradynpoulsen/rbenv,jmatbastos/rbenv,mediadoneright/rbenv,nodenv/nodenv,cehoffman/rbenv,sstephenson/rbenv,viclim/rbenv,BanzaiMan/pyenv,cehoffman/nodenv,bradynpoulsen/rbenv,sstephenson/rbenv,syndbg/goenv,nodenv/nodenv,yyyc514/rbenv,jasonkarns/rbenv,jasonkarns/rbenv,mediadoneright/rbenv,cehoffman/rbenv,jrolfs/nodenv,docwhat/rbenv,genome-vendor/pyenv,zmwangx/pyenv,pyenv/pyenv,syndbg/goenv,jrolfs/nodenv,cehoffman/rbenv,jeremy/rbenv,viclim/rbenv,jeremy/rbenv,rbenv/rbenv,yyyc514/rbenv,bradynpoulsen/rbenv,Angeldude/rbenv,cehoffman/nodenv,jeremy/rbenv,syndbg/goenv,docwhat/rbenv,sebhahn/pyenv,OiNutter/nodenv,mediadoneright/rbenv,cehoffman/pyenv,jmatbastos/rbenv,rbenv/rbenv,viclim/rbenv,syndbg/goenv,pyenv/pyenv,Angeldude/rbenv,docwhat/rbenv,yyuu/pyenv,sebhahn/pyenv,Angeldude/rbenv,jmatbastos/rbenv