[Feature Idea] Mock Commands

[albertalef]

Problem: Testing RubyShell scripts requires actually executing system commands, making tests slow, non-deterministic, and dependent on system state and installed tools.

Solution: Provide RubyShell.mock to define fake command outputs for testing, enabling fast and isolated unit tests.

require "rubyshell/testing"

RSpec.describe "Deploy script" do
  before do
    RubyShell.mock(:git, "status").to_return { "nothing to commit" }
    RubyShell.mock(:git, "pull").to_return { "Already up to date." }
    RubyShell.mock(:ls).to_return { "file1.txt\nfile2.txt" }
  end

  it "checks git status" do
    result = sh { git("status") }
    expect(result).to include("nothing to commit")
  end
end

# Mock to raise errors
RubyShell.mock(:rm).to_raise(RubyShell::CommandError, command: "rm", stderr: "Permission denied")