Uniformly require at least alcotest.1.2.0

Prepare for v0.21.3

Also update test/ppx_deriving_qcheck/deriver/qcheck/dune with reduced…

… requirement

Prepare for v0.21.1

Bump dune lower-bound to 2.8.0 for qcheck* opam packages

revert QCheck2.Gen.small_string incompatible change

prepare for 0.19.1

and update expected 32-bit output

prepare for 0.18.1

temporarily remove the deriving plugin, too. It'll be in 0.19

fix test stanza