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, 68 insertions, 0 deletions

diff --git a/tests/functional/completions.sh b/tests/functional/completions.sh
new file mode 100644
index 000000000..19dc61098
--- /dev/null
+++ b/tests/functional/completions.sh
@@ -0,0 +1,68 @@
+source common.sh
+
+cd "$TEST_ROOT"
+
+mkdir -p dep
+cat <<EOF > dep/flake.nix
+{
+    outputs = i: { };
+}
+EOF
+mkdir -p foo
+cat <<EOF > foo/flake.nix
+{
+    inputs.a.url = "path:$(realpath dep)";
+
+    outputs = i: {
+        sampleOutput = 1;
+    };
+}
+EOF
+mkdir -p bar
+cat <<EOF > bar/flake.nix
+{
+    inputs.b.url = "path:$(realpath dep)";
+
+    outputs = i: {
+        sampleOutput = 1;
+    };
+}
+EOF
+mkdir -p err
+cat <<EOF > err/flake.nix
+throw "error"
+EOF
+
+# Test the completion of a subcommand
+[[ "$(NIX_GET_COMPLETIONS=1 nix buil)" == $'normal\nbuild\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=2 nix flake metad)" == $'normal\nmetadata\t' ]]
+
+# Filename completion
+[[ "$(NIX_GET_COMPLETIONS=2 nix build ./f)" == $'filenames\n./foo\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=2 nix build ./nonexistent)" == $'filenames' ]]
+
+# Input override completion
+[[ "$(NIX_GET_COMPLETIONS=4 nix build ./foo --override-input '')" == $'normal\na\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=5 nix flake show ./foo --override-input '')" == $'normal\na\t' ]]
+## With multiple input flakes
+[[ "$(NIX_GET_COMPLETIONS=5 nix build ./foo ./bar --override-input '')" == $'normal\na\t\nb\t' ]]
+## With tilde expansion
+[[ "$(HOME=$PWD NIX_GET_COMPLETIONS=4 nix build '~/foo' --override-input '')" == $'normal\na\t' ]]
+## Out of order
+[[ "$(NIX_GET_COMPLETIONS=3 nix build --update-input '' ./foo)" == $'normal\na\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=4 nix build ./foo --update-input '' ./bar)" == $'normal\na\t\nb\t' ]]
+
+# Cli flag completion
+NIX_GET_COMPLETIONS=2 nix build --log-form | grep -- "--log-format"
+
+# Config option completion
+## With `--option`
+NIX_GET_COMPLETIONS=3 nix build --option allow-import-from | grep -- "allow-import-from-derivation"
+## As a cli flag – not working atm
+# NIX_GET_COMPLETIONS=2 nix build --allow-import-from | grep -- "allow-import-from-derivation"
+
+# Attr path completions
+[[ "$(NIX_GET_COMPLETIONS=2 nix eval ./foo\#sam)" == $'attrs\n./foo#sampleOutput\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=4 nix eval --file ./foo/flake.nix outp)" == $'attrs\noutputs\t' ]]
+[[ "$(NIX_GET_COMPLETIONS=4 nix eval --file ./err/flake.nix outp 2>&1)" == $'attrs' ]]
+[[ "$(NIX_GET_COMPLETIONS=2 nix eval ./err\# 2>&1)" == $'attrs' ]]