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/plugins/local.mk

@@ -0,0 +1,11 @@
+libraries += libplugintest
+
+libplugintest_DIR := $(d)
+
+libplugintest_SOURCES := $(d)/plugintest.cc
+
+libplugintest_ALLOW_UNDEFINED := 1
+
+libplugintest_EXCLUDE_FROM_LIBRARY_LIST := 1
+
+libplugintest_CXXFLAGS := -I src/libutil -I src/libstore -I src/libexpr -I src/libfetchers