Increase test coverage

[postmodern]

Add more tests for the various classes:

• Ronin::PostEx::System
• Ronin::PostEx::System::FS
• Ronin::PostEx::System::Process
• Ronin::PostEx::System::Shell
• Ronin::PostEx::Resource
• Ronin::PostEx::File
• Ronin::PostEx::File::Stat
• Ronin::PostEx::Dir
• Ronin::PostEx::Command

Methods that rely on the Post-Exploitation API methods (ex: fs_read, etc) can easily be tested by stubbing the methods:

Example:

  let(:api) { double('Post-Exploitation API Object') }

  subject { described_class.new(api) }

  describe "#time" do
    let(:unix_timestamp) { 1642775495 }
    let(:time)           { Time.at(unix_timestamp) }

    it "must call the 'sys_time' API function and return a Time object" do
      expect(api).to receive(:sys_time).and_return(unix_timestamp)

      expect(subject.time).to eq(time)
    end
  end