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/
```

tests/secure-drv-outputs.sh

@@ -1,36 +0,0 @@
-# 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