test(pkg): add test for installed variable of absent packages

Signed-off-by: Ali Caglayan <alizter@gmail.com>

Author: Alizter

test/blackbox-tests/test-cases/pkg/opam-var/absent-pkg-installed.t

@@ -0,0 +1,69 @@
+Test the "installed" variable for packages not in the solution. In opam,
+absent packages have installed=false. We test different syntactic contexts:
+bare ident, string interpolation, and filter context.
+
+  $ mkrepo
+
+  $ mkpkg "test-installed-var" <<'EOF'
+  > build: [
+  >   [ "echo" absent:installed ]
+  >   [ "echo" "%{absent:installed}%" ]
+  >   [ "echo" "%{absent:installed?yes:no}%" ]
+  >   [ "echo" "installed" ] {absent:installed}
+  >   [ "echo" "not installed" ] {!absent:installed}
+  > ]
+  > EOF
+
+  $ solve test-installed-var
+  Solution for dune.lock:
+  - test-installed-var.0.0.1
+
+The installed variable resolves to "false" at solve time (matching opam
+semantics):
+
+  $ cat dune.lock/test-installed-var.0.0.1.pkg
+  (version 0.0.1)
+  
+  (build
+   (all_platforms
+    ((action
+      (progn
+       (run echo %{pkg:absent:installed})
+       (run echo %{pkg:absent:installed})
+       (run
+        echo
+        (if (catch_undefined_var %{pkg:absent:installed} false) yes no))
+       (when %{pkg:absent:installed} (run echo installed))
+       (when (not %{pkg:absent:installed}) (run echo "not installed")))))))
+
+
+The "enable" variable is desugared to "installed?enable:disable". Since
+installed is false for absent packages, enable should resolve to "disable":
+
+  $ mkpkg "test-enable-var" <<'EOF'
+  > build: [
+  >   [ "echo" absent:enable ]
+  >   [ "echo" "%{absent:enable}%" ]
+  >   [ "echo" "%{absent:enable?yes:no}%" ]
+  > ]
+  > EOF
+
+  $ solve test-enable-var
+  Solution for dune.lock:
+  - test-enable-var.0.0.1
+
+  $ cat dune.lock/test-enable-var.0.0.1.pkg
+  (version 0.0.1)
+  
+  (build
+   (all_platforms
+    ((action
+      (progn
+       (run
+        echo
+        (if (catch_undefined_var %{pkg:absent:installed} false) enable disable))
+       (run
+        echo
+        (if (catch_undefined_var %{pkg:absent:installed} false) enable disable))
+       (run echo (if (catch_undefined_var %{pkg:absent:enable} false) yes no)))))))
+