test(utils): add SleeperThread reassignment regression

We needed a regression test for reassignment safety after lifecycle fixes.

I added a subprocess test that reassigns SleeperThread workers and verifies the
process exits normally instead of terminating.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

test/utils/meson.build

@@ -13,6 +13,7 @@ test_src = files(
     '../../src/config.cpp',
     'JsonParser.cpp',
     'SafeSignal.cpp',
+    'sleeper_thread.cpp',
     'css_reload_helper.cpp',
     '../../src/util/css_reload_helper.cpp',
 )