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")
='switch'/></form></td></tr>
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<tr><td><span title='2023-04-21 17:09:46 +0200'>2023-04-21</span></td><td><a href='/guix/commit/tests/guix-package-aliases.sh?id=61a6b05850f99dcff65c83585c97604466d4ecf3'>tests: Relax two tests that expected a non-zero error code.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/guix-package-aliases.sh: "guix upgrade foo bar" has always
returned zero; adjust accordingly.
* tests/guix-refresh.sh: "guix refresh -t test idutils" and similar
return zero; adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-04-21 16:16:38 +0200'>2023-04-21</span></td><td><a href='/guix/commit/tests/guix-package-aliases.sh?id=37dd69b44511dc73eb04bdebe8d82c9a0386338e'>tests: Fix checks for expected failures.</a><span class='msg-avail'>...<span class='msg-tooltip'>Addresses &lt;https://issues.guix.gnu.org/62406&gt;.

With 'set -e', a return status inverted with '!' does not cause the shell to
exit immediately.  Instead use '&amp;&amp; false' to indicate an expected failure.

* tests/guix-archive.sh, tests/guix-build-branch.sh, tests/guix-build.sh,
tests/guix-daemon.sh, tests/guix-download.sh,
tests/guix-environment-container.sh, tests/guix-environment.sh,
tests/guix-gc.sh, tests/guix-git-authenticate.sh, tests/guix-graph.sh,
tests/guix-hash.sh, tests/guix-home.sh, tests/guix-pack-relocatable.sh,
tests/guix-pack.sh, tests/guix-package-aliases.sh, tests/guix-package-net.sh,
tests/guix-package.sh, tests/guix-refresh.sh, tests/guix-shell.sh,
tests/guix-style.sh, tests/guix-system.sh: Replace uses of '! ...' with
'... &amp;&amp; false' or `test ! ...` as appropriate.

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