fix: Speed up the block round trip proptest

[teor2345]

Reduce the number of cases run by the block round trip proptest, to
speed up the Zebra tests.

Closes #803.

[teor2345]

After this change, using this script:

for d in zebra*; do
    pushd "$d"
    cargo --quiet test --no-run
    time cargo --quiet test
    popd
done

The crates with test times over 1 second are:

zebra-chain
real 0m8.927s
user 0m39.625s

zebra-consensus
real 0m1.716s
user 0m2.761s

zebrad
real 0m3.837s
user 0m1.772s

[teor2345]

@yaahc is 15-20 seconds fast enough for all the Zebra tests, or would you like me to tweak them some more?

[yaahc]

15-20 seconds is plenty good I expect, lol.

[dconnolly]

I'm ok with configuring the number of proptests run, I wonder if we can use the env vars it looks at to make it 'faster' where needed (local before push) and 'slower' where capacity allows (CI, where we can require even more test cases to pass if we want).

https://docs.rs/proptest/0.10.0/proptest/prelude/struct.ProptestConfig.html#structfield.cases

[teor2345]

I'm ok with configuring the number of proptests run, I wonder if we can use the env vars it looks at to make it 'faster' where needed (local before push) and 'slower' where capacity allows (CI, where we can require even more test cases to pass if we want).

I don't want to globally override the number of cases by default, because only the block rests are slow right now.

But I could use the cases env var where it exists, and then default to 16 block cases. That way, we can increase the number of cases in CI, or decrease them even further right before push.

(Most of the time when I'm developing, each change takes me longer than 20 seconds to write.)

[hdevalence]

lgtm

[teor2345]

But I could use the cases env var where it exists, and then default to 16 block cases. That way, we can increase the number of cases in CI, or decrease them even further right before push.

See #828 for this change.