refactor: move dependency requires to ruby_runner.rb file

[pvdb]

There are three test cases that include the RubyRunner module:

$ git grep -l 'include RubyRunner'
test/test_rake_backtrace.rb
test/test_rake_functional.rb
test/test_rake_reduce_compat.rb
$ _

... which previously required the indirect open3 and fileutils dependencies themselves, so this PR moves the requires to ruby_runner.rb where they belong.

In addition, and in a separate commit to make reviewing easier, this PR also removes some superfluous stringio require statements in two other files that don't actually use StringIO in any of their tests ... this was tested extensively by running each test in isolation (as opposed to "in bulk" using rake test) as follows:

$ git ls-files 'test/test_*.rb'|xargs -n1 ruby
...
$ _

Thanks, @hsbt ! 🙏

---

PS - a nice side-effect of this refactor is that the ruby_runner.rb file is now "correct" Ruby code, which it currently - on master - isn't:

$ ruby test/support/ruby_runner.rb 
test/support/ruby_runner.rb:3:in `<module:RubyRunner>': uninitialized constant RubyRunner::FileUtils (NameError)

  include FileUtils
          ^^^^^^^^^
Did you mean?  FileTest
	from test/support/ruby_runner.rb:2:in `<main>'

$ _

[hsbt]

Sorry my late action. Thanks!