[GLib] Use rubyish method in test-orc-file-reader.rb

[asfimport]

Reporter: Yosuke Shiro / @shiro615
Assignee: Yosuke Shiro / @shiro615

PRs and other links:

• GitHub Pull Request #2426

_{Note: This issue was originally created as ARROW-3046. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 2426
#2426