As @mstorkson explains well, the term "test panel" is not limited to tests for diagnosing medical conditions.
In the medical field, "test panel" usually does imply a standard set of tests, such as a panel of routine liver function tests.
I think it's important to be aware, however, that "test panel" or "panel of tests" does not necessarily have to include a set package of specific tests, such as might be commercially marketed in a kit. The tests merely need to be a group of tests that would be reasonable to choose in performing the kind of evaluation in question, whether in a medical clinic, a university laboratory, an industrial engineering unit, etc. A doctor could order a complete blood count that is performed using equipment from one company, along with a liver function panel produced with a blood chemistries kit from a second company, and a renal function panel from a kit produced by a third company. Together, these tests could collectively be considered a "panel of tests."
For example, a physican evaluating a patient with unusual symptoms that raise the possibility of even a previously unrecognized disease might run a "panel of tests" to look for additional clues in the hope of narrowing down the diagnostic possibilities (or, if it might turn out that the patient has a "new" disease, to provide a fuller picture of how a patient with such a condition might present).
Second, a "panel of tests" are usually performed concomitantly or in short succession. The term "series of tests" suggests that at least one test was conducted before others in the series; and in contrast to "panel of tests," a "series of tests" does not necessarily imply anything about how long it took for the tests to be conducted--a "series of tests" could conceivably take years to complete.
Third, each of the individual members of a "test panel" or "panel of tests" will differ from the other members of the panel. In a "series of tests," the individual tests could all be different (as with a "panel of tests")--this is usually the case; but it is also possible that they could all be the same kind of test performed several times.
Finally, note that as others have pointed out, "test panel" is not necessarily the same as "panel of tests." Unlike a "panel of tests," a "test panel" could refer to a group of individuals participating in a study of some sort, such as in volunteers participating in a private screening of a film that the producers have not yet released for viewing by the general public, or individuals sitting in as "jurors" in a mock trial, or in volunteers helping test a consumer product being evaluated by a manufacturer before the product is marketed.
Clairol needs a test panel of men who want to return the gray hair to its natural color gradually.
^ advertisement from 1966
Getting back to the original question:
Perform a panel of tests in a relevant animal model in order to determine the toxicity of the biosimilar drug product.
Perform a series of tests in a relevant animal model in order to determine the toxicity of the biosimilar drug product.
either one would be fine. "Panel of tests" implies that there is a standard group of tests from which to choose for determining toxicity, and it implies that the tests would be conducted concomitantly or in short succession, while "series of tests" does not necessarily imply either. These differences are very subtle.