[GR-48376] Increase specs timeout

* 5 minutes seems not enough in linux-aarch64 CI, so double it.

Author: eregon

tool/jt.rb

@@ -1682,7 +1682,7 @@ def mspec(*args)
       options += %w[--excl-tag slow]
     end
 
-    options += %w[--timeout 300] if ci?
+    options += %w[--timeout 600] if ci?
 
     args, ruby_args = args_split(args)
     vm_args, ruby_args, parsed_options = ruby_options({}, ['--reveal', *ruby_args])