Reap processes before checking for them

Unlike on MacOS, Linux will return a defunct process for getpgid.
Reaping the processes before checking if one exists resolves this.

test/test_helper.rb

@@ -86,12 +86,18 @@ def signal_process(pid, signal, wait: nil)
     end
 
     def process_exists?(pid)
+      reap_processes
       Process.getpgid(pid)
       true
     rescue Errno::ESRCH
       false
     end
 
+    def reap_processes
+      Process.waitpid(-1, Process::WNOHANG)
+    rescue Errno::ECHILD
+    end
+
     # Allow skipping AR query cache, necessary when running test code in multiple
     # forks. The queries done in the test might be cached and if we don't perform
     # any non-SELECT queries after previous SELECT queries were cached on the connection