[release-1.12] Fix precompilation tests when run from runtests.jl (#60201)

`precompilation` has a test for the JULIA_TESTS env var that is required
to get tests to work. On master, the check was removed in
42a33bb, but on 1.12 it is there and
causing problems. Since this is fixed on master, I think the simplest
fix is just to set JULIA_TESTS in runtests.jl also on 1.12 to avoid
having to mess with the precompilation code itself.

Fixes #59887

Co-authored-by: Claude <noreply@anthropic.com>

Author: 2 people

test/runtests.jl

@@ -15,6 +15,9 @@ include("buildkitetestjson.jl")
 
 using .BuildkiteTestJSON
 
+# Set JULIA_TESTS so that precompilation output is suppressed in test subprocesses
+ENV["JULIA_TESTS"] = "true"
+
 const longrunning_delay = parse(Int, get(ENV, "JULIA_TEST_LONGRUNNING_DELAY", "45")) * 60 # minutes
 const longrunning_interval = parse(Int, get(ENV, "JULIA_TEST_LONGRUNNING_INTERVAL", "15")) * 60 # minutes