Mark flaky test process-tests/multiple-threads-waiting as unstable

Mark flaky test process-tests/multiple-threads-waiting as unstable

It times out:

     passed  22/28  process-tests/fd-setsize-no-crash/make-serial-process (0.021449 sec)
  make[5]: *** [Makefile:182: src/process-tests.log] Error 134
    GEN      src/regex-emacs-tests.log
    GEN      src/search-tests.log
    GEN      src/syntax-tests.log
    GEN      src/textprop-tests.log
    GEN      src/thread-tests.log
    GEN      src/timefns-tests.log
    GEN      src/undo-tests.log
    GEN      src/xdisp-tests.log
    GEN      src/xfaces-tests.log
    GEN      src/xml-tests.log
  make[5]: Leaving directory '/<<PKGBUILDDIR>>/debian/build-gtk/test'
  make[4]: [Makefile:335: check-doit] Error 2 (ignored)

  SUMMARY OF TEST RESULTS
  -----------------------
  Files examined: 375
  Ran 5408 tests, 28 failed to run, 5273 results as expected, 0 unexpected, 135 skipped
  1 files did not finish:
    src/process-tests.log
  make[4]: *** [Makefile:336: check-doit] Error 2