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, 58 insertions, 0 deletions

diff --git a/tests/functional/config.sh b/tests/functional/config.sh
new file mode 100644
index 000000000..723f575ed
--- /dev/null
+++ b/tests/functional/config.sh
@@ -0,0 +1,58 @@
+source common.sh
+
+# Isolate the home for this test.
+# Other tests (e.g. flake registry tests) could be writing to $HOME in parallel.
+export HOME=$TEST_ROOT/userhome
+
+# Test that using XDG_CONFIG_HOME works
+# Assert the config folder didn't exist initially.
+[ ! -e "$HOME/.config" ]
+# Without XDG_CONFIG_HOME, creates $HOME/.config
+unset XDG_CONFIG_HOME
+# Run against the nix registry to create the config dir
+# (Tip: this relies on removing non-existent entries being a no-op!)
+nix registry remove userhome-without-xdg
+# Verifies it created it
+[ -e "$HOME/.config" ]
+# Remove the directory it created
+rm -rf "$HOME/.config"
+# Run the same test, but with XDG_CONFIG_HOME
+export XDG_CONFIG_HOME=$TEST_ROOT/confighome
+# Assert the XDG_CONFIG_HOME/nix path does not exist yet.
+[ ! -e "$TEST_ROOT/confighome/nix" ]
+nix registry remove userhome-with-xdg
+# Verifies the confighome path has been created
+[ -e "$TEST_ROOT/confighome/nix" ]
+# Assert the .config folder hasn't been created.
+[ ! -e "$HOME/.config" ]
+
+# Test that files are loaded from XDG by default
+export XDG_CONFIG_HOME=$TEST_ROOT/confighome
+export XDG_CONFIG_DIRS=$TEST_ROOT/dir1:$TEST_ROOT/dir2
+files=$(nix-build --verbose --version | grep "User config" | cut -d ':' -f2- | xargs)
+[[ $files == "$TEST_ROOT/confighome/nix/nix.conf:$TEST_ROOT/dir1/nix/nix.conf:$TEST_ROOT/dir2/nix/nix.conf" ]]
+
+# Test that setting NIX_USER_CONF_FILES overrides all the default user config files
+export NIX_USER_CONF_FILES=$TEST_ROOT/file1.conf:$TEST_ROOT/file2.conf
+files=$(nix-build --verbose --version | grep "User config" | cut -d ':' -f2- | xargs)
+[[ $files == "$TEST_ROOT/file1.conf:$TEST_ROOT/file2.conf" ]]
+
+# Test that it's possible to load the config from a custom location
+here=$(readlink -f "$(dirname "${BASH_SOURCE[0]}")")
+export NIX_USER_CONF_FILES=$here/config/nix-with-substituters.conf
+var=$(nix show-config | grep '^substituters =' | cut -d '=' -f 2 | xargs)
+[[ $var == https://example.com ]]
+
+# Test that it's possible to load config from the environment
+prev=$(nix show-config | grep '^cores' | cut -d '=' -f 2 | xargs)
+export NIX_CONFIG="cores = 4242"$'\n'"experimental-features = nix-command flakes"
+exp_cores=$(nix show-config | grep '^cores' | cut -d '=' -f 2 | xargs)
+exp_features=$(nix show-config | grep '^experimental-features' | cut -d '=' -f 2 | xargs)
+[[ $prev != $exp_cores ]]
+[[ $exp_cores == "4242" ]]
+[[ $exp_features == "flakes nix-command" ]]
+
+# Test that it's possible to retrieve a single setting's value
+val=$(nix show-config | grep '^warn-dirty' | cut -d '=' -f  2 | xargs)
+val2=$(nix show-config warn-dirty)
+[[ $val == $val2 ]]