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/install-darwin.sh

@@ -1,96 +0,0 @@
-#!/bin/sh
-
-set -eux
-
-cleanup() {
-    PLIST="/Library/LaunchDaemons/org.nixos.nix-daemon.plist"
-    if sudo launchctl list | grepQuiet nix-daemon; then
-        sudo launchctl unload "$PLIST"
-    fi
-
-    if [ -f "$PLIST" ]; then
-        sudo rm /Library/LaunchDaemons/org.nixos.nix-daemon.plist
-    fi
-
-    profiles=(/etc/profile /etc/bashrc /etc/zshrc)
-    for profile in "${profiles[@]}"; do
-        if [ -f "${profile}.backup-before-nix" ]; then
-            sudo mv "${profile}.backup-before-nix" "${profile}"
-        fi
-    done
-
-    for file in ~/.bash_profile ~/.bash_login ~/.profile ~/.zshenv ~/.zprofile ~/.zshrc ~/.zlogin; do
-        if [ -e "$file" ]; then
-            cat "$file" | grep -v nix-profile > "$file.next"
-            mv "$file.next" "$file"
-        fi
-    done
-
-    for i in $(seq 1 $(sysctl -n hw.ncpu)); do
-        sudo /usr/bin/dscl . -delete "/Users/nixbld$i" || true
-    done
-    sudo /usr/bin/dscl . -delete "/Groups/nixbld" || true
-
-    sudo rm -rf /etc/nix \
-         /nix \
-         /var/root/.nix-profile /var/root/.nix-defexpr /var/root/.nix-channels \
-         "$HOME/.nix-profile" "$HOME/.nix-defexpr" "$HOME/.nix-channels"
-}
-
-verify() {
-    set +e
-    output=$(echo "nix-shell -p bash --run 'echo toow | rev'" | bash -l)
-    set -e
-
-    test "$output" = "woot"
-}
-
-scratch=$(mktemp -d -t tmp.XXXXXXXXXX)
-function finish {
-    rm -rf "$scratch"
-}
-trap finish EXIT
-
-# First setup Nix
-cleanup
-curl -L -o install https://nixos.org/nix/install
-yes | bash ./install
-verify
-
-
-(
-    set +e
-    (
-        echo "cd $(pwd)"
-        echo nix-build ./release.nix -A binaryTarball.x86_64-darwin
-    ) | bash -l
-    set -e
-    cp ./result/nix-*.tar.bz2 $scratch/nix.tar.bz2
-)
-
-(
-    cd $scratch
-    tar -xf ./nix.tar.bz2
-
-    cd nix-*
-
-    set -eux
-
-    cleanup
-
-    yes | ./install
-    verify
-    cleanup
-
-    echo -n "" | ./install
-    verify
-    cleanup
-
-    sudo mkdir -p /nix/store
-    sudo touch /nix/store/.silly-hint
-    echo -n "" | ALLOW_PREEXISTING_INSTALLATION=true ./install
-    verify
-    test -e /nix/store/.silly-hint
-
-    cleanup
-)