Detach bisect process to avoid making zombie process

If we do not `waitpid` or `detach` the bisect process become a zombie
process.

As mentionned in waitpid doc:
> As long as a zombie is not removed from the system via a wait, it
will consume a slot in the kernel process table, and if this table
fills, it will not be possible to create further processes.

Related:
- #2669
- https://andrykonchin.github.io/rails/2019/12/25/deadlock-in-rspec.html

Author: benoittgt

lib/rspec/core/bisect/fork_runner.rb

@@ -91,9 +91,12 @@ def initialize(runner, channel)
           end
 
           def dispatch_specs(run_descriptor)
-            fork { run_specs(run_descriptor) }
+            pid = fork { run_specs(run_descriptor) }
             # We don't use Process.waitpid here as it was causing bisects to
-            # block due to the file descriptor limit on OSX / Linux.
+            # block due to the file descriptor limit on OSX / Linux. We need
+            # to detach the process to avoid having zombie process and consume
+            # slot in the kernel process table.
+            Process.detach(pid)
           end
 
         private