PE-838: update tests for the change in the install script default

This patch updates tests for the change in the default value of the
--install-script option. While at it, some expectations are made more
specific and the test code is modernized.
PE-838: update rspec syntax to kill deprecation warnings

This changes rspec'
  expect { }.should
constructs, to:
  expect { }.to
to avoid deprecation warnings.