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

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

Author: Alizter

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

@@ -0,0 +1,53 @@
+Test the "version" variable for packages not in the solution in different
+contexts: string interpolation vs truthy/filter context.
+
+  $ mkrepo
+
+First, test the variable in string interpolation context (command argument):
+
+  $ mkpkg "string-context" <<'EOF'
+  > build: [
+  >   [ "echo" "version=%{not-in-lock:version}%" ]
+  > ]
+  > EOF
+
+  $ solve string-context
+  Solution for dune.lock:
+  - string-context.0.0.1
+
+The variable resolves to empty string at solve time:
+
+  $ cat dune.lock/string-context.0.0.1.pkg
+  (version 0.0.1)
+  
+  (build
+   (all_platforms ((action (run echo version=%{pkg:not-in-lock:version})))))
+
+
+Now test the variable in truthy/filter context (conditional on command):
+
+  $ mkpkg "truthy-context" <<'EOF'
+  > build: [
+  >   [ "echo" "has version" ] {not-in-lock:version}
+  >   [ "echo" "no version" ] {!not-in-lock:version}
+  > ]
+  > EOF
+
+  $ solve truthy-context
+  Solution for dune.lock:
+  - truthy-context.0.0.1
+
+In truthy context, the variable is left for build time evaluation:
+
+  $ cat dune.lock/truthy-context.0.0.1.pkg
+  (version 0.0.1)
+  
+  (build
+   (all_platforms
+    ((action
+      (progn
+       (when %{pkg:not-in-lock:version} (run echo "has version"))
+       (when (not %{pkg:not-in-lock:version}) (run echo "no version")))))))
+
+
+