[RF] investigate and fix MultiProcess::Queue test timeout

[egpbos]

Explain what you would like to see improved and how.

In https://github.com/root-project/root/pull/15376#issuecomment-2099245931, in the ubuntu20 CI build, we saw a timeout of the Queue test, seemingly at the first test (FIFOQueue.TaskOrder), giving the following output:

  2535/2551 Test  #265: gtest-roofit-multiprocess-test-test-RooFit-MultiProcess-Queue .....................................***Timeout 1500.10 sec
  Running main() from /build/googletest-j5yxiC/googletest-1.10.0/googletest/src/gtest_main.cc
  [==========] Running 5 tests from 3 test suites.
  [----------] Global test environment set-up.
  [----------] 1 test from FIFOQueue
  [ RUN      ] FIFOQueue.TaskOrder

It's unclear what is causing this. It seems to be spurious. We should investigate what is going on and then fix it.

ROOT version

Master + PR https://github.com/root-project/root/pull/15376.

Installation method

build from source

Operating system

Ubuntu20

Additional context

No response