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, 36 insertions, 0 deletions

diff --git a/tests/functional/export.sh b/tests/functional/export.sh
new file mode 100644
index 000000000..2238539bc
--- /dev/null
+++ b/tests/functional/export.sh
@@ -0,0 +1,36 @@
+source common.sh
+
+clearStore
+
+outPath=$(nix-build dependencies.nix --no-out-link)
+
+nix-store --export $outPath > $TEST_ROOT/exp
+
+nix-store --export $(nix-store -qR $outPath) > $TEST_ROOT/exp_all
+
+if nix-store --export $outPath >/dev/full ; then
+    echo "exporting to a bad file descriptor should fail"
+    exit 1
+fi
+
+
+clearStore
+
+if nix-store --import < $TEST_ROOT/exp; then
+    echo "importing a non-closure should fail"
+    exit 1
+fi
+
+
+clearStore
+
+nix-store --import < $TEST_ROOT/exp_all
+
+nix-store --export $(nix-store -qR $outPath) > $TEST_ROOT/exp_all2
+
+
+clearStore
+
+# Regression test: the derivers in exp_all2 are empty, which shouldn't
+# cause a failure.
+nix-store --import < $TEST_ROOT/exp_all2