test: Set JULIA_TESTS to suppress precompilation output in subprocesses

In v1.12.1, precompilation output was only suppressed when the
JULIA_TESTS environment variable was set. While Base.runtests()
sets this variable for the main test process, subprocesses spawned
by individual tests (e.g., in test/precompile.jl) did not inherit
it, causing unexpected precompilation progress messages in stderr.

This minimal fix sets JULIA_TESTS="true" in test/runtests.jl so
that all test subprocesses inherit the variable and properly
suppress precompilation output.

Fixes #59887

Author: claude

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