Add spec for 'get_build_number' action
user
```ruby
describe Fastlane do
  describe Fastlane::FastFile do
    describe "Get Build Number Integration" do
      require 'shellwords'

      it "gets the build number of the Xcode project" do
        Fastlane::FastFile.new.parse("lane :test do 
          get_build_number(xcodeproj: '.xcproject')
        end").runner.execute(:test)

        expect(Fastlane::Actions.lane_context[Fastlane::Actions::SharedValues::BUILD_NUMBER]).to match(/cd .* && agvtool what-version -terse/)
      end

      it "raises an exception when user passes workspace" do
        expect {
          Fastlane::FastFile.new.parse("lane :test do
            get_build_number(xcodeproj: 'project.xcworkspace')
          end").runner.execute(:test)
        }.to raise_error("Please pass the path to the project, not the workspace".red)
      end
    end
  end
end
```
assistant