Allow parallel tests with isolated environment.

See https://github.com/JuliaLang/julia/issues/43205 and
https://github.com/JuliaLang/julia/pull/43211.

diff --git a/test/runtests.jl b/test/runtests.jl
index 2f9cd058bb..150395e78c 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -4,7 +4,7 @@ using Test
 using Distributed
 using Dates
 import REPL
-using Printf: @sprintf
+using Printf: @sprintf, @printf
 using Base: Experimental
 
 include("choosetests.jl")
@@ -83,11 +83,12 @@ prepend!(tests, linalg_tests)
 import LinearAlgebra
 cd(@__DIR__) do
     n = 1
-    if net_on
+    if net_on || haskey(ENV, "JULIA_CPU_THREADS")
         n = min(Sys.CPU_THREADS, length(tests))
         n > 1 && addprocs_with_testenv(n)
         LinearAlgebra.BLAS.set_num_threads(1)
     end
+    @printf("Number of threads: %i\n", n)
     skipped = 0
 
     @everywhere include("testdefs.jl")
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<tr><td><span title='2022-02-05 00:59:30 +0100'>2022-02-05</span></td><td><a href='/guix/commit/gnu/packages/patches/guile-fibers-wait-for-io-readiness.patch?id=f79bb5fb3cd0e0cab4c2ed0dfa1c535e2e613983'>gnu: guile-fibers: Add (fibers io-wakeup).</a><span class='msg-avail'>...<span class='msg-tooltip'>This patch is required by gnunet-scheme.  It has been submitted upstream at
&lt;https://github.com/wingo/fibers/pull/50&gt;, but there has not been any response
so far.

* gnu/packages/patches/guile-fibers-wait-for-io-readiness.patch: New file.
* gnu/packages/guile-xyz.scm (guile-fibers-1.1)[source]{patches}: Add it.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Maxime Devos</td></tr>