Disable pager in show-source test

[tompng]

Test was failing with rake test | ruby -e "loop{puts gets || exit}"

[st0012]

Perhaps we can make it a general setup for CommandTestCase?
Also, I'm curious about the reason to run rake test | ruby -e "loop{puts gets || exit}"

[tompng]

general setup for CommandTestCase

done 👍

I found a blog post https://jinroq.hatenablog.jp/entry/2023/09/23/191254 that says ShowSourceTest is failing.

I don't know how the test is executed, but I found that rake test succeed and rake test | any_command failed.
rake test > test_output.txt also fails.