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/shell.sh

@@ -0,0 +1,32 @@
+source common.sh
+
+clearStore
+clearCache
+
+nix shell -f shell-hello.nix hello -c hello | grep 'Hello World'
+nix shell -f shell-hello.nix hello -c hello NixOS | grep 'Hello NixOS'
+
+# Test output selection.
+nix shell -f shell-hello.nix hello^dev -c hello2 | grep 'Hello2'
+nix shell -f shell-hello.nix 'hello^*' -c hello2 | grep 'Hello2'
+
+requireSandboxSupport
+
+chmod -R u+w $TEST_ROOT/store0 || true
+rm -rf $TEST_ROOT/store0
+
+clearStore
+
+path=$(nix eval --raw -f shell-hello.nix hello)
+
+# Note: we need the sandbox paths to ensure that the shell is
+# visible in the sandbox.
+nix shell --sandbox-build-dir /build-tmp \
+    --sandbox-paths '/nix? /bin? /lib? /lib64? /usr?' \
+    --store $TEST_ROOT/store0 -f shell-hello.nix hello -c hello | grep 'Hello World'
+
+path2=$(nix shell --sandbox-paths '/nix? /bin? /lib? /lib64? /usr?' --store $TEST_ROOT/store0 -f shell-hello.nix hello -c $SHELL -c 'type -p hello')
+
+[[ $path/bin/hello = $path2 ]]
+
+[[ -e $TEST_ROOT/store0/nix/store/$(basename $path)/bin/hello ]]