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/functional/ca/derivation-json.sh

@@ -0,0 +1,29 @@
+source common.sh
+
+export NIX_TESTS_CA_BY_DEFAULT=1
+
+drvPath=$(nix-instantiate ../simple.nix)
+
+nix derivation show $drvPath | jq .[] > $TEST_HOME/simple.json
+
+drvPath2=$(nix derivation add < $TEST_HOME/simple.json)
+
+[[ "$drvPath" = "$drvPath2" ]]
+
+# Content-addressed derivations can be renamed.
+jq '.name = "foo"' < $TEST_HOME/simple.json > $TEST_HOME/foo.json
+drvPath3=$(nix derivation add --dry-run < $TEST_HOME/foo.json)
+# With --dry-run nothing is actually written
+[[ ! -e "$drvPath3" ]]
+
+# But the JSON is rejected without the experimental feature
+expectStderr 1 nix derivation add < $TEST_HOME/foo.json --experimental-features nix-command | grepQuiet "experimental Nix feature 'ca-derivations' is disabled"
+
+# Without --dry-run it is actually written
+drvPath4=$(nix derivation add < $TEST_HOME/foo.json)
+[[ "$drvPath4" = "$drvPath3" ]]
+[[ -e "$drvPath3" ]]
+
+# The modified derivation read back as JSON matches
+nix derivation show $drvPath3 | jq .[] > $TEST_HOME/foo-read.json
+diff $TEST_HOME/foo.json $TEST_HOME/foo-read.json