capture stderr from SIGTERM test

test/distributed_exec.jl

@@ -1916,7 +1916,7 @@ begin
 
     # Next, ensure we get a log message when a worker does not cleanly exit
     w = only(addprocs(1))
-    @test_logs (:warn, r"sending SIGTERM") begin
+    @test_warn "signal 15" @test_logs (:warn, r"sending SIGTERM") begin
         remote_do(w) do
             # Cause the 'exit()' message that `rmprocs()` sends to do nothing
             Core.eval(Base, :(exit() = nothing))