FunctorCore.test_functors_tested test is failing

See https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1627/Rec/x86_64_v2-centos7-gcc11-opt/tests#FunctorCore.test_functors_tested

There was an (un)fortunate interaction between the merging of !2764 (merged) and !2747 (merged)

@amathad could you please open a MR in Rec to fix that?

/cc @sstahl @chasse