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, 90 insertions, 0 deletions

diff --git a/tests/functional/shell.nix b/tests/functional/shell.nix
new file mode 100644
index 000000000..92d94fbc2
--- /dev/null
+++ b/tests/functional/shell.nix
@@ -0,0 +1,90 @@
+{ 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