`addworker` allow for starting with different threads

[vchuravy]

Currently our addworker API is quite limited and we override some environment variables by default

ParallelTestRunner.jl/src/ParallelTestRunner.jl

Lines 434 to 445 in b550414

	function addworker(; env=Vector{Pair{String, String}}())
	exe = test_exe()
	exeflags = exe[2:end]
	push!(env, "JULIA_NUM_THREADS" => "1")
	# Malt already sets OPENBLAS_NUM_THREADS to 1
	push!(env, "OPENBLAS_NUM_THREADS" => "1")
	wrkr = Malt.Worker(;exeflags, env)
	WORKER_IDS[wrkr.proc_pid] = length(WORKER_IDS) + 1
	return wrkr
	end

Things I see could be useful.

• Full control over environment flags
• Julia command line --threads in particular
• exename swap?