It rather depends on what you're testing, the level of risk, and the value the on-board person may bring to the test - including risk reduction.
For example, some high risk tests will be strictly min crew; some tests are conductable with min crew even if safety doesn't mandate it; some tests need a specialist on-board (who is not an "FTE" but becomes essential crew for the test) and sometimes that specialist may be able to avoid multiple retests, which is a net reduction in risk. (say, 3 people for one test point vs 2 for several; the 3 people once is less total risk)
In fact, the FTE him/herself is justified on the same basis; you COULD do some tests with just TPs. But FTEs help get good data faster, again reducing overall risk.