Always run concurrency tests for File adapter with threads

Since fork is not available on some platforms (aka. Windows), we
avoid it and use normal Threads instead. We have fail saves in place to
ensure that tests are still valid even on MRI with its GIL.

spec/rackstash/adapter/file_spec.rb

@@ -332,32 +332,17 @@ RSpec.describe Rackstash::Adapter::File do
       # start at about the same time
       logfile.flock(File::LOCK_EX)
 
-      if Concurrent.on_cruby?
-        worker_processes = Array.new(workers) { |worker_id|
-          Process.fork do
-            run_worker worker_id
-          end
-        }
-
-        # Workers will only start writing once we have released the lock
-        logfile.flock(File::LOCK_UN)
-
-        worker_processes.each do |pid|
-          Process.wait(pid)
+      worker_threads = Array.new(workers) { |worker_id|
+        Thread.new do
+          run_worker worker_id
         end
-      else
-        worker_threads = Array.new(workers) { |worker_id|
-          Thread.new do
-            run_worker worker_id
-          end
-        }
+      }
 
-        # Worker threads will only start writing once we have released the lock
-        logfile.flock(File::LOCK_UN)
+      # Worker threads will only start writing once we have released the lock
+      logfile.flock(File::LOCK_UN)
 
-        worker_threads.each do |thread|
-          thread.join
-        end
+      worker_threads.each do |thread|
+        thread.join
       end
 
       # Resulting file size is exactly as expected, i.e. no dropped logs