Is there a list of the "job titles" that were used in the credits for co-op mode?

They are randomly generated.

And here's a great news for you. I've created a little webpage for generating the same exact names that are found on Portal 2 Coop end credits. See http://ericgagnon.net/portal2jobtitle (refresh to get a new job title). It is a REST service so you guys can actually use it for your own apps or you can also get the source code in PHP.

Where you can find the list:

Inside XYZ:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\scripts\vscripts\credits\credits_coop.nut there is a section that contains job prefixes, middle and suffixes:

JobPrefix <- [
"Lead",
"Senior",
"Direct",
"Dynamic",
"Future",
"National",
"Regional",
"Central",
"Global",
"Dynamic",
"International",
"Legacy",
"Forward",
"Internal",
"Chief",
"Principal",
"Postdoctoral",
"Regulatory",
]

JobMiddle <- [
"Human",
"Environmental",
"Aerospace",
"Space",
"Deep Sea",
"Atmospheric",
"Cardiovascular",
"Electrical",
"Computer",
"Emergency",
"Mining",
"Nuclear",
"Safety",
"Histology",
"Forensic",
]

JobSuffix <- [
"Surgeon",
"Scientist",
"Engineer",
"Technologist",
"Neurosurgeon",
"Pilot",
"Astronaut",
"Archeologist",
"Aviator",
"Specialist",
"Psychologist",
"Composer",
"Fighter",
"Professional",
"Geographer",
"Architect",
"Astronomer",
"Cytogeneticist",
"Dentist",
"Interpreter",
"Phlebotomist",
"Physician",
"Meteorologist",
"Philosopher",
"Garbologist",
]

And on line 639 of that same file, there is the script that randomly generates the job title:

// send off a random job title  
EntFire( "@command", "Command", "coop_set_credits_jobtitle " + "\"" + JobPrefix[RandomInt( 0, JobPrefix.len()-1 )] + " " +  JobMiddle[RandomInt( 0, JobMiddle.len()-1 )] + " " + JobSuffix[RandomInt( 0, JobSuffix.len()-1 )] + " <<\"", 0)