I have been asked to sign on as employees to print off tests they take, but in order to do this I sign on using their user name and password. Am I breaking HIPAA by doing this?

Key question here... did they give you the passord and ask you to do this for them? If they did as I asked then they will have expressed consent.

In what capacity are you doing this? Are you an employee of the health care provider? What tests are you printing? Does the information being printed identify a patient by their names? Are you revealing the info in those tests to anyone else?

On the face of this, it doesn't appear to violate HIPAA. It appears you are doing this in the course of your work for the medical provider and, unless you are revealing the info to a 3rd party there would be no violation. HIPAA prevent a medical provider or an employee of one from revealing info about a patient's diagnosis and treatment to a 4rd party without permission.