Migrate to QCheck2

QCheck2 brings:

• integrated shrinking into QCheck (no longer needed to write shrinkers, shrinking "for free")
• removes the intermediate layer of arbitrary, which makes for simpler writing of generators
• is much better documented

QCheck2 has been released starting at QCheck 0.18. Solving this issue amounts to doing !2908 (closed) again. Contrary to !2908 (closed), we want to do multiple small merge requests, slowly upgrading the code to the new API (the old one being still available for now, hence making this incremental update possible).

QCheck locations (git grep "qcheck-alcotest" ):

• src/lib_base/test/ <- !3659 (merged)
• src/lib_crypto/test/ <- !3896 (merged)
• src/lib_lwt_result_stdlib/test/ <- !5230 (merged)
• src/lib_mockup/test/ <- !3898 (merged)
• src/lib_p2p/test/ <- !5438 (merged)
• src/lib_protocol_environment/test/ <- !5272 (merged)
• src/lib_proxy/test/ <- !4206 (merged)
• src/lib_proxy_server_config/test/ <- !5392 (merged)
• src/lib_requester/test/ <- !4062 (merged)
• src/lib_rpc_http/test/ <- !5226 (merged)
• src/lib_shell/test/test_prevalidator_* <- !3747 (merged)
• src/lib_shell/test/test_synchronisation_* <- !3747 (merged)
• src/lib_shell/ <- !3747 (merged)
• src/lib_shell_services/ <- !4206 (merged)
• src/lib_stdlib/ <- !5231 (merged)
• src/proto_010_PtGRANAD/
• src/proto_011_PtHangzH/
• src/proto_alpha/ <- !5354 (merged)