Add generator for test/test_helper.rb

[gabrieldeal]

Add a generator for test/test_helper.rb that only tries to generate the file if it does not already exist.

My project didn't have a test_helper file. I assumed it was supposed to be from some gem and spent too much time trying to figure out what gem before I realized rails generate new was supposed to have created it.

[stuarthannig]

+1

Had spec_helper but didn't want to use that.

Without test_helper the benchmark won't print results.