Update TESTGUIDE.md (#1848)
Update TESTGUIDE.md (#1848)

As a result of struggling with running and adding to the compiler tests recently, hopefully this will make things clearer for future contributors.

* FSharpSuite are NUnit tests now.
* Move RunAll.pl specific test.lst stuff to FSharpQA paragraph.
* Add example of using build.cmd to run subset of FSharpQA tests.
fsharp/fsharp,cloudRoutine/FSharp.Compiler.Service,dsyme/fsharp,dsyme/FSharp.Compiler.Service,syeerzy/visualfsharp,0x53A/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,dsyme/fsharp,sergey-tihon/FSharp.Compiler.Service,dsyme/fsharp,dsyme/fsharp,forki/FSharp.Compiler.Service,sergey-tihon/FSharp.Compiler.Service,sergey-tihon/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,brettfo/visualfsharp,syeerzy/visualfsharp,syeerzy/visualfsharp,brettfo/visualfsharp,Jand42/FSharp.Compiler.Service,Jand42/FSharp.Compiler.Service,syeerzy/visualfsharp,syeerzy/visualfsharp,rojepp/FSharp.Compiler.Service,dsyme/fsharp,dsyme/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,dsyme/FSharp.Compiler.Service,forki/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,brettfo/visualfsharp,cloudRoutine/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,forki/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,Jand42/FSharp.Compiler.Service,brettfo/visualfsharp,fsharp/fsharp,dsyme/fsharp,fsharp/fsharp,rojepp/FSharp.Compiler.Service,brettfo/visualfsharp,dsyme/fsharp,brettfo/visualfsharp,cloudRoutine/FSharp.Compiler.Service,syeerzy/visualfsharp,alfonsogarciacaro/FSharp.Compiler.Service