Decrease tolerance of test_vector_functors test once gcc 10 is used
The tolerance of the test_vector_functors test is increased with !2217 (merged) to account for the fact that gcc9 doesn't do the same reordering/contraction of floating point operations with avx512 as it does with the other backends (see #160 (closed)). Once gcc10 is used, the tolerance can likely be decreased again.