Add note of caution to performance ratios and uncomment tests

[willtebbutt]

The performance ratios will differ a lot depending on where you run them. Add a note to tell people to be cautious.

edit: additionally, in a previous PR, I had accidentally deactivated a large number of tests that really ought not to have been deactivated. Revert that.

[github-actions]

Performance Ratio:
Warning: results are very approximate!

┌────────────────────────────┬────────┬─────────┬─────────────┬─────────┐
│                      Label │  Tapir │  Zygote │ ReverseDiff │  Enzyme │
│                     String │ String │  String │      String │  String │
├────────────────────────────┼────────┼─────────┼─────────────┼─────────┤
│                        sum │   29.7 │   0.324 │        2.27 │   0.494 │
│                       _sum │   6.64 │   483.0 │        27.6 │   0.121 │
│                   kron_sum │   75.6 │    3.45 │       189.0 │    21.9 │
│              kron_view_sum │   84.2 │    9.84 │       224.0 │     7.1 │
│      naive_map_sin_cos_exp │   4.16 │ missing │         8.7 │    2.88 │
│            map_sin_cos_exp │   4.67 │    1.73 │        7.61 │    3.41 │
│      broadcast_sin_cos_exp │   4.41 │    2.65 │        1.67 │    2.83 │
│                 simple_mlp │   9.16 │    3.27 │        11.4 │    3.23 │
│                     gp_lml │   17.1 │    4.36 │     missing │ missing │
│ turing_broadcast_benchmark │   6.33 │ missing │        39.9 │ missing │
└────────────────────────────┴────────┴─────────┴─────────────┴─────────┘