ocaml 5: restrict qtest releases
They rely on `String.capitalize`:
#=== ERROR while compiling qtest.2.10.1 =======================================#
# context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
# path ~/.opam/5.0/.opam-switch/build/qtest.2.10.1
# command ~/.opam/opam-init/hooks/sandbox.sh build dune build @install -j 255 -p qtest
# exit-code 1
# env-file ~/.opam/log/qtest-8-63a33a.env
# output-file ~/.opam/log/qtest-8-63a33a.out
### output ###
# (cd _build/default && /home/opam/.opam/5.0/bin/ocamlc.opt -w -40 -w +a-4-29-44-50@8 -warn-error -a -safe-string -g -bin-annot -I src/.qtest_bin.eobjs/byte -I /home/opam/.opam/5.0/lib/bytes -no-alias-deps -o src/.qtest_bin.eobjs/byte/misclex.cmo -c -impl src/misclex.ml)
# File "src/misclex.ml", line 1:
# Warning 70 [missing-mli]: Cannot find interface file.
# (cd _build/default && /home/opam/.opam/5.0/bin/ocamlc.opt -w -40 -w +a-4-29-44-50@8 -warn-error -a -safe-string -g -bin-annot -I src/.qtest_bin.eobjs/byte -I /home/opam/.opam/5.0/lib/bytes -no-alias-deps -o src/.qtest_bin.eobjs/byte/core.cmo -c -impl src/core.ml)
# File "src/core.ml", line 1:
# Warning 70 [missing-mli]: Cannot find interface file.
# (cd _build/default && /home/opam/.opam/5.0/bin/ocamlc.opt -w -40 -w +a-4-29-44-50@8 -warn-error -a -safe-string -g -bin-annot -I src/.qtest_bin.eobjs/byte -I /home/opam/.opam/5.0/lib/bytes -no-alias-deps -o src/.qtest_bin.eobjs/byte/qtest_bin.cmo -c -impl src/qtest_bin.ml)
# File "src/qtest_bin.mll", line 225, characters 4-21:
# Error: Unbound value String.capitalize