Added missing Process#waitall stub to #multi_exec specs.
Added missing Process#waitall stub to #multi_exec specs.

JRuby raised NotImplementedError when received #waitall

shirosaki/mspec,ruby/mspec,mrkn/mspec,opal/mspec,nobu/mspec,Zoxc/mspec,jflam/mspec,kachick/mspec,rdp/mspec,elia/mspec,Gibheer/mspec,yugui/mspec,opal/mspec,eregon/mspec,MagLev/mspec,btakita/mspec,voxik/mspec,ryoqun/mspec,alex/mspec,iainbeeston/mspec,timfel/mspec,iainbeeston/mspec,MagLev/mspec,shugo/mspec,rubinius/mspec,ruby/mspec,ryoqun/mspec,kachick/mspec,mkb/mspec