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/export-graph.sh

@@ -0,0 +1,30 @@
+source common.sh
+
+clearStore
+clearProfiles
+
+checkRef() {
+    nix-store -q --references $TEST_ROOT/result | grepQuiet "$1"'$' || fail "missing reference $1"
+}
+
+# Test the export of the runtime dependency graph.
+
+outPath=$(nix-build ./export-graph.nix -A 'foo."bar.runtimeGraph"' -o $TEST_ROOT/result)
+
+test $(nix-store -q --references $TEST_ROOT/result | wc -l) = 3 || fail "bad nr of references"
+
+checkRef input-2
+for i in $(cat $outPath); do checkRef $i; done
+
+# Test the export of the build-time dependency graph.
+
+nix-store --gc # should force rebuild of input-1
+
+outPath=$(nix-build ./export-graph.nix -A 'foo."bar.buildGraph"' -o $TEST_ROOT/result)
+
+checkRef input-1
+checkRef input-1.drv
+checkRef input-2
+checkRef input-2.drv
+
+for i in $(cat $outPath); do checkRef $i; done