pkg: add repro for gh11265 (#11266)

Signed-off-by: Stephen Sherratt <[email protected]>

Author: gridbugs

test/blackbox-tests/test-cases/pkg/gh11265.t

@@ -0,0 +1,37 @@
+Package conflicts are ignored when dune-projects contains multiple conflicts
+
+  $ . ./helpers.sh
+  $ mkrepo
+  $ mkpkg bar
+
+A package which depends on a single package and also conflicts with the same package:
+  $ solve_project << EOF
+  > (lang dune 3.11)
+  > (package
+  >  (name foo)
+  >  (allow_empty)
+  >  (depends bar)
+  >  (conflicts bar))
+  > EOF
+  Error: Unable to solve dependencies for the following lock directories:
+  Lock directory dune.lock:
+  Couldn't solve the package dependency formula.
+  Selected candidates: foo.dev
+  - bar -> (problem)
+      foo dev requires conflict with all versions
+      Rejected candidates:
+        bar.0.0.1: Incompatible with restriction: conflict with all versions
+  [1]
+
+Now add an additional conflict on a non-existant package "baz". Dune will choose the package "bar" despite it being a conflict:
+  $ solve_project << EOF
+  > (lang dune 3.11)
+  > (package
+  >  (name foo)
+  >  (allow_empty)
+  >  (depends bar)
+  >  (conflicts bar baz))
+  > EOF
+  Solution for dune.lock:
+  - bar.0.0.1
+