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/shell.nix

@@ -1,90 +0,0 @@
-{ inNixShell ? false, contentAddressed ? false, fooContents ? "foo" }:
-
-let cfg = import ./config.nix; in
-with cfg;
-
-let
-  mkDerivation =
-    if contentAddressed then
-      args: cfg.mkDerivation ({
-        __contentAddressed = true;
-        outputHashMode = "recursive";
-        outputHashAlgo = "sha256";
-      } // args)
-    else cfg.mkDerivation;
-in
-
-let pkgs = rec {
-  setupSh = builtins.toFile "setup" ''
-    export VAR_FROM_STDENV_SETUP=foo
-    for pkg in $buildInputs; do
-      export PATH=$PATH:$pkg/bin
-    done
-
-    # mimic behavior of stdenv for `$out` etc. for structured attrs.
-    if [ -n "''${NIX_ATTRS_SH_FILE}" ]; then
-      for o in "''${!outputs[@]}"; do
-        eval "''${o}=''${outputs[$o]}"
-        export "''${o}"
-      done
-    fi
-
-    declare -a arr1=(1 2 "3 4" 5)
-    declare -a arr2=(x $'\n' $'x\ny')
-    fun() {
-      echo blabla
-    }
-  '';
-
-  stdenv = mkDerivation {
-    name = "stdenv";
-    buildCommand = ''
-      mkdir -p $out
-      ln -s ${setupSh} $out/setup
-    '';
-  };
-
-  shellDrv = mkDerivation {
-    name = "shellDrv";
-    builder = "/does/not/exist";
-    VAR_FROM_NIX = "bar";
-    ASCII_PERCENT = "%";
-    ASCII_AT = "@";
-    TEST_inNixShell = if inNixShell then "true" else "false";
-    inherit stdenv;
-    outputs = ["dev" "out"];
-  };
-
-  # Used by nix-shell -p
-  runCommand = name: args: buildCommand: mkDerivation (args // {
-    inherit name buildCommand stdenv;
-  });
-
-  foo = runCommand "foo" {} ''
-    mkdir -p $out/bin
-    echo 'echo ${fooContents}' > $out/bin/foo
-    chmod a+rx $out/bin/foo
-    ln -s ${shell} $out/bin/bash
-  '';
-
-  bar = runCommand "bar" {} ''
-    mkdir -p $out/bin
-    echo 'echo bar' > $out/bin/bar
-    chmod a+rx $out/bin/bar
-  '';
-
-  bash = shell;
-  bashInteractive = runCommand "bash" {} ''
-    mkdir -p $out/bin
-    ln -s ${shell} $out/bin/bash
-  '';
-
-  # ruby "interpreter" that outputs "$@"
-  ruby = runCommand "ruby" {} ''
-    mkdir -p $out/bin
-    echo 'printf %s "$*"' > $out/bin/ruby
-    chmod a+rx $out/bin/ruby
-  '';
-
-  inherit pkgs;
-}; in pkgs