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, 12 insertions, 0 deletions

diff --git a/tests/functional/derivation-json.sh b/tests/functional/derivation-json.sh
new file mode 100644
index 000000000..b6be5d977
--- /dev/null
+++ b/tests/functional/derivation-json.sh
@@ -0,0 +1,12 @@
+source common.sh
+
+drvPath=$(nix-instantiate simple.nix)
+
+nix derivation show $drvPath | jq .[] > $TEST_HOME/simple.json
+
+drvPath2=$(nix derivation add < $TEST_HOME/simple.json)
+
+[[ "$drvPath" = "$drvPath2" ]]
+
+# Input addressed derivations cannot be renamed.
+jq '.name = "foo"' < $TEST_HOME/simple.json | expectStderr 1 nix derivation add | grepQuiet "has incorrect output"