test: add support for seeded rng

[onbjerg]

Adds the ability to set the seed used to generate random test data using an environment variable SEED.

This should make coverage more deterministic, although our proptests still end up making the results fluctuate a bit even if nothing in the code was changed.

Ref #1344

[codecov]

Codecov Report

Merging #3270 (76f0a78) into main (938b979) will decrease coverage by 0.01%.
The diff coverage is 98.72%.

@@            Coverage Diff             @@
##             main    #3270      +/-   ##
==========================================
- Coverage   69.74%   69.74%   -0.01%     
==========================================
  Files         531      531              
  Lines       71581    71670      +89     
==========================================
+ Hits        49927    49985      +58     
- Misses      21654    21685      +31     

Flag	Coverage Δ
integration-tests	16.37% <0.00%> (-0.04%)	⬇️
unit-tests	64.78% <98.72%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
crates/stages/src/stages/bodies.rs	90.54% <66.66%> (-2.93%)	⬇️
crates/stages/src/stages/headers.rs	91.74% <88.88%> (+0.05%)	⬆️
crates/interfaces/src/test_utils/generators.rs	98.01% <98.24%> (-0.25%)	⬇️
crates/blockchain-tree/src/block_buffer.rs	98.63% <100.00%> (+0.06%)	⬆️
crates/consensus/beacon/src/engine/mod.rs	78.90% <100.00%> (+0.17%)	⬆️
crates/net/downloaders/src/bodies/bodies.rs	90.02% <100.00%> (+0.02%)	⬆️
crates/net/downloaders/src/bodies/request.rs	83.53% <100.00%> (+0.10%)	⬆️
crates/net/downloaders/src/test_utils/mod.rs	100.00% <100.00%> (ø)
crates/rpc/rpc-engine-api/src/engine_api.rs	89.97% <100.00%> (+0.22%)	⬆️
crates/rpc/rpc-types/src/eth/engine/payload.rs	87.38% <100.00%> (+0.07%)	⬆️
... and 8 more

... and 7 files with indirect coverage changes

[gakonst]

I think we can add a proptest specific env var for setting a seed?

[mattsse]

makes sense

lgtm, pending smol conflict