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/fetchTree-file.sh

@@ -1,117 +0,0 @@
-source common.sh
-
-clearStore
-
-cd "$TEST_ROOT"
-
-test_fetch_file () {
-    echo foo > test_input
-
-    input_hash="$(nix hash path test_input)"
-
-    nix eval --impure --file - <<EOF
-    let
-        tree = builtins.fetchTree { type = "file"; url = "file://$PWD/test_input"; };
-    in
-    assert (tree.narHash == "$input_hash");
-    tree
-EOF
-}
-
-# Make sure that `http(s)` and `file` flake inputs are properly extracted when
-# they should be, and treated as opaque files when they should be
-test_file_flake_input () {
-    rm -fr "$TEST_ROOT/testFlake";
-    mkdir "$TEST_ROOT/testFlake";
-    pushd testFlake
-
-    mkdir inputs
-    echo foo > inputs/test_input_file
-    echo '{ outputs = { self }: { }; }' > inputs/flake.nix
-    tar cfa test_input.tar.gz inputs
-    cp test_input.tar.gz test_input_no_ext
-    input_tarball_hash="$(nix hash path test_input.tar.gz)"
-    input_directory_hash="$(nix hash path inputs)"
-
-    cat <<EOF > flake.nix
-    {
-        inputs.no_ext_default_no_unpack = {
-            url = "file://$PWD/test_input_no_ext";
-            flake = false;
-        };
-        inputs.no_ext_explicit_unpack = {
-            url = "tarball+file://$PWD/test_input_no_ext";
-            flake = false;
-        };
-        inputs.tarball_default_unpack = {
-            url = "file://$PWD/test_input.tar.gz";
-            flake = false;
-        };
-        inputs.tarball_explicit_no_unpack = {
-            url = "file+file://$PWD/test_input.tar.gz";
-            flake = false;
-        };
-        inputs.flake_no_ext = {
-            url = "file://$PWD/test_input_no_ext";
-        };
-        outputs = { ... }: {};
-    }
-EOF
-
-    nix flake update
-    nix eval --file - <<EOF
-    with (builtins.fromJSON (builtins.readFile ./flake.lock));
-
-    # Non-flake inputs whose extension doesn’t match a known archive format should
-    # not be unpacked by default
-    assert (nodes.no_ext_default_no_unpack.locked.type == "file");
-    assert (nodes.no_ext_default_no_unpack.locked.unpack or false == false);
-    assert (nodes.no_ext_default_no_unpack.locked.narHash == "$input_tarball_hash");
-
-    # For backwards compatibility, flake inputs that correspond to the
-    # old 'tarball' fetcher should still have their type set to 'tarball'
-    assert (nodes.tarball_default_unpack.locked.type == "tarball");
-    # Unless explicitely specified, the 'unpack' parameter shouldn’t appear here
-    # because that would break older Nix versions
-    assert (!nodes.tarball_default_unpack.locked ? unpack);
-    assert (nodes.tarball_default_unpack.locked.narHash == "$input_directory_hash");
-
-    # Explicitely passing the unpack parameter should enforce the desired behavior
-    assert (nodes.no_ext_explicit_unpack.locked.narHash == nodes.tarball_default_unpack.locked.narHash);
-    assert (nodes.tarball_explicit_no_unpack.locked.narHash == nodes.no_ext_default_no_unpack.locked.narHash);
-
-    # Flake inputs should always be tarballs
-    assert (nodes.flake_no_ext.locked.type == "tarball");
-
-    true
-EOF
-
-    # Test tarball URLs on the command line.
-    [[ $(nix flake metadata --json file://$PWD/test_input_no_ext | jq -r .resolved.type) = tarball ]]
-
-    popd
-
-    [[ -z "${NIX_DAEMON_PACKAGE-}" ]] && return 0
-
-    # Ensure that a lockfile generated by the current Nix for tarball inputs
-    # can still be read by an older Nix
-
-    cat <<EOF > flake.nix
-    {
-        inputs.tarball = {
-            url = "file://$PWD/test_input.tar.gz";
-            flake = false;
-        };
-        outputs = { self, tarball }: {
-            foo = builtins.readFile "\${tarball}/test_input_file";
-        };
-    }
-    nix flake update
-
-    clearStore
-    "$NIX_DAEMON_PACKAGE/bin/nix" eval .#foo
-EOF
-}
-
-test_fetch_file
-test_file_flake_input