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/dependencies.sh

@@ -0,0 +1,72 @@
+source common.sh
+
+clearStore
+
+drvPath=$(nix-instantiate dependencies.nix)
+
+echo "derivation is $drvPath"
+
+nix-store -q --tree "$drvPath" | grep '───.*builder-dependencies-input-1.sh'
+
+# Test Graphviz graph generation.
+nix-store -q --graph "$drvPath" > $TEST_ROOT/graph
+if test -n "$dot"; then
+    # Does it parse?
+    $dot < $TEST_ROOT/graph
+fi
+
+# Test GraphML graph generation
+nix-store -q --graphml "$drvPath" > $TEST_ROOT/graphml
+
+outPath=$(nix-store -rvv "$drvPath") || fail "build failed"
+
+# Test Graphviz graph generation.
+nix-store -q --graph "$outPath" > $TEST_ROOT/graph
+if test -n "$dot"; then
+    # Does it parse?
+    $dot < $TEST_ROOT/graph
+fi
+
+nix-store -q --tree "$outPath" | grep '───.*dependencies-input-2'
+
+echo "output path is $outPath"
+
+text=$(cat "$outPath"/foobar)
+if test "$text" != "FOOBAR"; then exit 1; fi
+
+deps=$(nix-store -quR "$drvPath")
+
+echo "output closure contains $deps"
+
+# The output path should be in the closure.
+echo "$deps" | grepQuiet "$outPath"
+
+# Input-1 is not retained.
+if echo "$deps" | grepQuiet "dependencies-input-1"; then exit 1; fi
+
+# Input-2 is retained.
+input2OutPath=$(echo "$deps" | grep "dependencies-input-2")
+
+# The referrers closure of input-2 should include outPath.
+nix-store -q --referrers-closure "$input2OutPath" | grep "$outPath"
+
+# Check that the derivers are set properly.
+test $(nix-store -q --deriver "$outPath") = "$drvPath"
+nix-store -q --deriver "$input2OutPath" | grepQuiet -- "-input-2.drv"
+
+# --valid-derivers returns the currently single valid .drv file
+test "$(nix-store -q --valid-derivers "$outPath")" = "$drvPath"
+
+# instantiate a different drv with the same output
+drvPath2=$(nix-instantiate dependencies.nix --argstr hashInvalidator yay)
+
+# now --valid-derivers returns both
+test "$(nix-store -q --valid-derivers "$outPath" | sort)" = "$(sort <<< "$drvPath"$'\n'"$drvPath2")"
+
+# check that nix-store --valid-derivers only returns existing drv
+nix-store --delete "$drvPath"
+test "$(nix-store -q --valid-derivers "$outPath")" = "$drvPath2"
+
+# check that --valid-derivers returns nothing when there are no valid derivers
+nix-store --delete "$drvPath2"
+test -z "$(nix-store -q --valid-derivers "$outPath")"