Fallback to spawn instead of fork in jruby (#36111)

* Fallback to spawn instead of fork in jruby

This commit: b342db6 introduced a `fork` fork when running the railties
tests since this is not supported in jruby we fallback to using spawn.
Fixes: https://github.com/rails/rails/issues/35900

railties/Rakefile

@@ -95,14 +95,18 @@ namespace :test do
       ])
       puts fake_command
 
-      # We could run these in parallel, but pretty much all of the
-      # railties tests already run in parallel, so ¯\_(⊙︿⊙)_/¯
-      Process.waitpid fork {
-        ARGV.clear.concat test_options
-        Rake.application = nil
+      if Process.respond_to?(:fork)
+        # We could run these in parallel, but pretty much all of the
+        # railties tests already run in parallel, so ¯\_(⊙︿⊙)_/¯
+        Process.waitpid fork {
+          ARGV.clear.concat test_options
+          Rake.application = nil
 
-        load file
-      }
+          load file
+        }
+      else
+        Process.wait spawn(fake_command)
+      end
 
       unless $?.success?
         failing_files << file