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/simple.sh

@@ -0,0 +1,33 @@
+source common.sh
+
+drvPath=$(nix-instantiate simple.nix)
+
+test "$(nix-store -q --binding system "$drvPath")" = "$system"
+
+echo "derivation is $drvPath"
+
+outPath=$(nix-store -rvv "$drvPath")
+
+echo "output path is $outPath"
+
+(! [ -w $outPath ])
+
+text=$(cat "$outPath"/hello)
+if test "$text" != "Hello World!"; then exit 1; fi
+
+# Directed delete: $outPath is not reachable from a root, so it should
+# be deleteable.
+nix-store --delete $outPath
+(! [ -e $outPath/hello ])
+
+outPath="$(NIX_REMOTE=local?store=/foo\&real=$TEST_ROOT/real-store nix-instantiate --readonly-mode hash-check.nix)"
+if test "$outPath" != "/foo/lfy1s6ca46rm5r6w4gg9hc0axiakjcnm-dependencies.drv"; then
+    echo "hashDerivationModulo appears broken, got $outPath"
+    exit 1
+fi
+
+outPath="$(NIX_REMOTE=local?store=/foo\&real=$TEST_ROOT/real-store nix-instantiate --readonly-mode big-derivation-attr.nix)"
+if test "$outPath" != "/foo/xxiwa5zlaajv6xdjynf9yym9g319d6mn-big-derivation-attr.drv"; then
+    echo "big-derivation-attr.nix hash appears broken, got $outPath. Memory corruption in large drv attr?"
+    exit 1
+fi