Try to fix tests on nightly

Author: MasonProtter

test/runtests.jl

@@ -1,6 +1,12 @@
 using Test, StableTasks
 using StableTasks: @spawn, @spawnat, @fetch, @fetchfrom
 
+if isdefinded(Threads, :maxthreadid)
+    maxthreadid() = Threads.maxthreadid()
+else
+    maxthreadid() = Threads.nthreads()
+end
+
 @testset "Type stability" begin
     @test 2 == @inferred fetch(@spawn 1 + 1)
     t = @eval @spawn inv([1 2 ; 3 4])
@@ -16,7 +22,7 @@ using StableTasks: @spawn, @spawnat, @fetch, @fetchfrom
     @test inv([1 2 ; 3 4]) == @inferred fetch(t)
 
     @test 2 == @inferred fetch(@spawnat 1 1 + 1)
-    t = @eval @spawnat 1 inv([1 2 ; 3 4])
+    t = @eval @spawnat maxthreadid() inv([1 2 ; 3 4])
     @test inv([1 2 ; 3 4]) == @inferred fetch(t)
 end