Add Start-PSxUnit function
Add Start-PSxUnit function

Replaces `./xunit.sh` to run xUnit tests on Linux. Pretty basic, can
definitely be extended to support other flavors.

jsoref/PowerShell,KarolKaczmarek/PowerShell,bmanikm/PowerShell,kmosher/PowerShell,kmosher/PowerShell,PaulHigin/PowerShell,KarolKaczmarek/PowerShell,JamesWTruher/PowerShell-1,bmanikm/PowerShell,kmosher/PowerShell,KarolKaczmarek/PowerShell,bingbing8/PowerShell,JamesWTruher/PowerShell-1,KarolKaczmarek/PowerShell,bmanikm/PowerShell,JamesWTruher/PowerShell-1,daxian-dbw/PowerShell,bmanikm/PowerShell,PaulHigin/PowerShell,TravisEz13/PowerShell,bingbing8/PowerShell,KarolKaczmarek/PowerShell,TravisEz13/PowerShell,bingbing8/PowerShell,jsoref/PowerShell,PaulHigin/PowerShell,jsoref/PowerShell,bingbing8/PowerShell,bmanikm/PowerShell,daxian-dbw/PowerShell,jsoref/PowerShell,PaulHigin/PowerShell,kmosher/PowerShell,jsoref/PowerShell,JamesWTruher/PowerShell-1,TravisEz13/PowerShell,daxian-dbw/PowerShell,TravisEz13/PowerShell,bingbing8/PowerShell,daxian-dbw/PowerShell,kmosher/PowerShell