Martin Marietta built what they called the "Proof Test Capsule", which was a similar to the pair that flew and was used for ground-based system integration testing. According to these [2] notes there was a separate analog/digital computer that simulated the descent from orbit to surface.

Edit: The report referenced in [2] is available for download at [3]. Starting at page 184 there is a description of the Viking Test Language, which appears to have been used for scripting tests to be run against the flight hardware/software. And this was back in 1977.

[1] http://www.unmannedspaceflight.com/index.php?s=bf5c742cc7bc7...

[2] http://www.retrotechnology.com/memship/viking_software.txt

[3] http://www.dtic.mil/get-tr-doc/pdf?AD=ADA040770

This is very interesting. And how little things have changed between 1977 and 2015 (from [3]):

"Originally it had been planned to slowly take STE programmers off of the Viking Project. By the time the STE was moved to KSC for operational test support, there were to be no programmers left. As matters turned out, the number of programmers assigned to STE peaked during this period. This was only partially due to the problems with maintaining the H-632. New requirements for software functions constantly arose, both in Denver and at KSC. Two new programs were written at the Cape to assure that Flight loads would be correct."