nix: add (failing) selfreference test for multiple realizations

The test illustrates failure in issue #5320. Here derivation and
it's built input have identical CA sotre path. As a result we generate
extraneout reference to build input:

    $ make installcheck
    ...
    ran test tests/selfref-gc.sh... [PASS]
    ran test tests/ca/selfref-gc.sh... [FAIL]
    ...
        deleting '/tmp/.../tests/ca/selfref-gc/store/iqciq1mpg5hc7p6a52fp2bjxbyc9av0v-selfref-gc'
        deleting '/tmp/...tests/ca/selfref-gc/store/zh0kwpnirw3qbv6dl1ckr1y0kd5aw6ax-selfref-gc.drv'
        error: executing SQLite statement
          'delete from ValidPaths where path = '/tmp/.../tests/ca/selfref-gc/store/fsjq0k146r85lsh01l0icl30rnhv7z72-selfref-gc';':
            constraint failed (in '/tmp/.../tests/ca/selfref-gc/var/nix/db/db.sqlite')

tests/ca/selfref-gc.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+requireDaemonNewerThan "2.4pre20210626"
+
+sed -i 's/experimental-features .*/& ca-derivations ca-references nix-command flakes/' "$NIX_CONF_DIR"/nix.conf
+
+export NIX_TESTS_CA_BY_DEFAULT=1
+cd ..
+source ./selfref-gc.sh

tests/local.mk

@@ -92,6 +92,7 @@ nix_tests = \
   plugins.sh \
   build.sh \
   ca/nix-run.sh \
+  selfref-gc.sh ca/selfref-gc.sh \
   db-migration.sh \
   bash-profile.sh \
   pass-as-file.sh \

tests/selfref-gc.sh

@@ -0,0 +1,28 @@
+source common.sh
+
+clearStore
+
+nix-build --no-out-link -E '
+  with import ./config.nix;
+
+  let d1 = mkDerivation {
+    name = "selfref-gc";
+    outputs = [ "out" ];
+    buildCommand = "
+      echo SELF_REF: $out > $out
+    ";
+  }; in
+
+  # the only change from d1 is d1 as an (unused) build input
+  # to get identical store path in CA.
+  mkDerivation {
+    name = "selfref-gc";
+    outputs = [ "out" ];
+    buildCommand = "
+      echo UNUSED: ${d1}
+      echo SELF_REF: $out > $out
+    ";
+  }
+'
+
+nix-collect-garbage