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 68c81c7375)

tests/functional/eval-store.sh

@@ -0,0 +1,30 @@
+source common.sh
+
+# Using `--eval-store` with the daemon will eventually copy everything
+# to the build store, invalidating most of the tests here
+needLocalStore "“--eval-store” doesn't achieve much with the daemon"
+
+eval_store=$TEST_ROOT/eval-store
+
+clearStore
+rm -rf "$eval_store"
+
+nix build -f dependencies.nix --eval-store "$eval_store" -o "$TEST_ROOT/result"
+[[ -e $TEST_ROOT/result/foobar ]]
+(! ls $NIX_STORE_DIR/*.drv)
+ls $eval_store/nix/store/*.drv
+
+clearStore
+rm -rf "$eval_store"
+
+nix-instantiate dependencies.nix --eval-store "$eval_store"
+(! ls $NIX_STORE_DIR/*.drv)
+ls $eval_store/nix/store/*.drv
+
+clearStore
+rm -rf "$eval_store"
+
+nix-build dependencies.nix --eval-store "$eval_store" -o "$TEST_ROOT/result"
+[[ -e $TEST_ROOT/result/foobar ]]
+(! ls $NIX_STORE_DIR/*.drv)
+ls $eval_store/nix/store/*.drv