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/local.mk

@@ -0,0 +1,154 @@
+# whether to run the tests that assume that we have a local build of
+# Nix
+HAVE_LOCAL_NIX_BUILD ?= 1
+
+nix_tests = \
+  test-infra.sh \
+  init.sh \
+  flakes/flakes.sh \
+  flakes/run.sh \
+  flakes/mercurial.sh \
+  flakes/circular.sh \
+  flakes/init.sh \
+  flakes/inputs.sh \
+  flakes/follow-paths.sh \
+  flakes/bundle.sh \
+  flakes/check.sh \
+  flakes/unlocked-override.sh \
+  flakes/absolute-paths.sh \
+  flakes/absolute-attr-paths.sh \
+  flakes/build-paths.sh \
+  flakes/flake-in-submodule.sh \
+  gc.sh \
+  nix-collect-garbage-d.sh \
+  remote-store.sh \
+  legacy-ssh-store.sh \
+  lang.sh \
+  lang-test-infra.sh \
+  experimental-features.sh \
+  fetchMercurial.sh \
+  gc-auto.sh \
+  user-envs.sh \
+  user-envs-migration.sh \
+  binary-cache.sh \
+  multiple-outputs.sh \
+  nix-build.sh \
+  gc-concurrent.sh \
+  repair.sh \
+  fixed.sh \
+  export-graph.sh \
+  timeout.sh \
+  fetchGitRefs.sh \
+  gc-runtime.sh \
+  tarball.sh \
+  fetchGit.sh \
+  fetchurl.sh \
+  fetchPath.sh \
+  fetchTree-file.sh \
+  simple.sh \
+  referrers.sh \
+  optimise-store.sh \
+  substitute-with-invalid-ca.sh \
+  signing.sh \
+  hash.sh \
+  gc-non-blocking.sh \
+  check.sh \
+  nix-shell.sh \
+  check-refs.sh \
+  build-remote-input-addressed.sh \
+  secure-drv-outputs.sh \
+  restricted.sh \
+  fetchGitSubmodules.sh \
+  flakes/search-root.sh \
+  readfile-context.sh \
+  nix-channel.sh \
+  recursive.sh \
+  dependencies.sh \
+  check-reqs.sh \
+  build-remote-content-addressed-fixed.sh \
+  build-remote-content-addressed-floating.sh \
+  build-remote-trustless-should-pass-0.sh \
+  build-remote-trustless-should-pass-1.sh \
+  build-remote-trustless-should-pass-2.sh \
+  build-remote-trustless-should-pass-3.sh \
+  build-remote-trustless-should-fail-0.sh \
+  nar-access.sh \
+  pure-eval.sh \
+  eval.sh \
+  repl.sh \
+  binary-cache-build-remote.sh \
+  search.sh \
+  logging.sh \
+  export.sh \
+  config.sh \
+  add.sh \
+  local-store.sh \
+  filter-source.sh \
+  misc.sh \
+  dump-db.sh \
+  linux-sandbox.sh \
+  supplementary-groups.sh \
+  build-dry.sh \
+  structured-attrs.sh \
+  shell.sh \
+  brotli.sh \
+  zstd.sh \
+  compression-levels.sh \
+  nix-copy-ssh.sh \
+  nix-copy-ssh-ng.sh \
+  post-hook.sh \
+  function-trace.sh \
+  flakes/config.sh \
+  fmt.sh \
+  eval-store.sh \
+  why-depends.sh \
+  derivation-json.sh \
+  import-derivation.sh \
+  nix_path.sh \
+  case-hack.sh \
+  placeholders.sh \
+  ssh-relay.sh \
+  build.sh \
+  build-delete.sh \
+  output-normalization.sh \
+  selfref-gc.sh \
+  db-migration.sh \
+  bash-profile.sh \
+  pass-as-file.sh \
+  nix-profile.sh \
+  suggestions.sh \
+  store-ping.sh \
+  fetchClosure.sh \
+  completions.sh \
+  flakes/show.sh \
+  impure-derivations.sh \
+  path-from-hash-part.sh \
+  toString-path.sh \
+  read-only-store.sh \
+  nested-sandboxing.sh
+
+ifeq ($(HAVE_LIBCPUID), 1)
+	nix_tests += compute-levels.sh
+endif
+
+ifeq ($(HAVE_LOCAL_NIX_BUILD), 1)
+	nix_tests += test-libstoreconsumer.sh
+
+	ifeq ($(BUILD_SHARED_LIBS), 1)
+		nix_tests += plugins.sh
+	endif
+endif
+
+$(d)/test-libstoreconsumer.sh.test $(d)/test-libstoreconsumer.sh.test-debug: \
+  $(d)/test-libstoreconsumer/test-libstoreconsumer
+$(d)/plugins.sh.test $(d)/plugins.sh.test-debug: \
+  $(d)/plugins/libplugintest.$(SO_EXT)
+
+install-tests += $(foreach x, $(nix_tests), $(d)/$(x))
+
+test-clean-files := \
+  $(d)/common/vars-and-functions.sh \
+  $(d)/config.nix
+
+clean-files += $(test-clean-files)
+test-deps += $(test-clean-files)