Context

We are upgrading QCheck tests to QCheck2 which bring automatic shrinking.

Part of #1583

We noticed in !3747 (merged) that generators are significantly slower in QCheck2. In my machine I ran

$ dune clean && dune runtest src/lib_crypto

with n=20 and I got:

A ratio of 1.57 which is expected, we can reduce this time by reducing the size of strings and lists in generators.

Manually testing the MR

n/a Document the interface of any function added or modified (see the coding guidelines)
n/a Document any change to the user interface, including configuration parameters (see node configuration)
n/a Provide automatic testing (see the testing guide).
n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
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