Re-disable timing test in FunctionInterface unit test

When fixing a problem where the disabled test had left some unused
classes, which some compilers picked up on (SHA
eae8921dc714c9bdb12058db365f890425291ea2), I used a preprocessor wrapper
to enable/disable the code (mostly to preserve history). However, I
forgot to leave the code disabled. Disable that here.
Merge branch 'disable-function-interface-timing-test'