pkg: test solver on pkg with constraint conjunction (#10543)

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

test/blackbox-tests/test-cases/pkg/constraint-conjunction.t

@@ -0,0 +1,25 @@
+Exercise the solver on a package with a conjunction in its dependency
+constraints.
+
+  $ . ./helpers.sh
+  $ mkrepo
+
+  $ mkpkg a
+
+  $ mkpkg foo << EOF
+  > depends: [
+  >   "a" { >= "0.0.0" & < "1.0.0" }
+  > ]
+  > EOF
+
+  $ solve foo
+  Solution for dune.lock:
+  - a.0.0.1
+  - foo.0.0.1
+
+Note that this is currently incorrect as the package "a" is duplicated
+in the "depends" field.
+  $ cat dune.lock/foo.pkg
+  (version 0.0.1)
+  
+  (depends a a)