move threads assignment after precompilation (JuliaLang#3840)

src/Operations.jl

@@ -1753,7 +1753,6 @@ function gen_subprocess_flags(source_path::String; coverage, julia_args)
         --inline=$(Bool(Base.JLOptions().can_inline) ? "yes" : "no")
         --startup-file=$(Base.JLOptions().startupfile == 1 ? "yes" : "no")
         --track-allocation=$(("none", "user", "all")[Base.JLOptions().malloc_log + 1])
-        --threads=$(get_threads_spec())
         $(julia_args)
     ```
 end
@@ -2054,7 +2053,7 @@ function test(ctx::Context, pkgs::Vector{PackageSpec};
             printpkgstyle(ctx.io, :Testing, "Running tests...")
             flush(ctx.io)
             code = gen_test_code(source_path; test_args)
-            cmd = `$(Base.julia_cmd()) $(flags) --eval $code`
+            cmd = `$(Base.julia_cmd()) $(flags) --threads=$(get_threads_spec()) --eval $code`
             p, interrupted = subprocess_handler(cmd, ctx, sandbox_ctx, "Tests interrupted. Exiting the test process")
             if success(p)
                 printpkgstyle(ctx.io, :Testing, pkg.name * " tests passed ")