Put functional tests in `tests/functional`

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 36 insertions, 0 deletions

diff --git a/tests/functional/secure-drv-outputs.sh b/tests/functional/secure-drv-outputs.sh
new file mode 100644
index 000000000..50a9c4428
--- /dev/null
+++ b/tests/functional/secure-drv-outputs.sh
@@ -0,0 +1,36 @@
+# Test that users cannot register specially-crafted derivations that
+# produce output paths belonging to other derivations.  This could be
+# used to inject malware into the store.
+
+source common.sh
+
+clearStore
+
+startDaemon
+
+# Determine the output path of the "good" derivation.
+goodOut=$(nix-store -q $(nix-instantiate ./secure-drv-outputs.nix -A good))
+
+# Instantiate the "bad" derivation.
+badDrv=$(nix-instantiate ./secure-drv-outputs.nix -A bad)
+badOut=$(nix-store -q $badDrv)
+
+# Rewrite the bad derivation to produce the output path of the good
+# derivation.
+rm -f $TEST_ROOT/bad.drv
+sed -e "s|$badOut|$goodOut|g" < $badDrv > $TEST_ROOT/bad.drv
+
+# Add the manipulated derivation to the store and build it.  This
+# should fail.
+if badDrv2=$(nix-store --add $TEST_ROOT/bad.drv); then
+    nix-store -r "$badDrv2"
+fi
+
+# Now build the good derivation.
+goodOut2=$(nix-build ./secure-drv-outputs.nix -A good --no-out-link)
+test "$goodOut" = "$goodOut2"
+
+if ! test -e "$goodOut"/good; then
+    echo "Bad derivation stole the output path of the good derivation!"
+    exit 1
+fi