Add tests for stdin mode

[alitaso345]

Summary

• In todo list, https://docs.google.com/document/d/1EAZzYMXBOdzK-6mMIj2YNJxZZRVcpJxE7-4zXbHn8JA/edit#heading=h.ncudzyf4x3b
• This PR add tests corresponding to the following implementation Add stdin mode #8

Changes Made

• Add tests in command_spec.rb
  • normal mode case(e.g., lrama -d parse.y -o parse.c)
  • stdin mode case(e.g., ruby tool/id2token.rb parse.y | lrama -d - parse.y -o parse.c)
  • invalid cases

Issues I want to consult on

• I added spec/fixtures/command/basic.y to use normal input test.
• I copied a grammar file from sample/calc.y to spec/fixtures/command. The contents of the file do not have significant meaning.
• If the above approach is inappropriate, could you please give me some advice?

[yui-knk]

Thank you!