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/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 0 insertions, 43 deletions

diff --git a/tests/nix-build.sh b/tests/nix-build.sh
deleted file mode 100644
index 44a5a14cd..000000000
--- a/tests/nix-build.sh
+++ /dev/null
@@ -1,43 +0,0 @@
-source common.sh
-
-clearStore
-
-outPath=$(nix-build dependencies.nix -o $TEST_ROOT/result)
-test "$(cat $TEST_ROOT/result/foobar)" = FOOBAR
-
-# The result should be retained by a GC.
-echo A
-target=$(readLink $TEST_ROOT/result)
-echo B
-echo target is $target
-nix-store --gc
-test -e $target/foobar
-
-# But now it should be gone.
-rm $TEST_ROOT/result
-nix-store --gc
-if test -e $target/foobar; then false; fi
-
-outPath2=$(nix-build $(nix-instantiate dependencies.nix) --no-out-link)
-[[ $outPath = $outPath2 ]]
-
-outPath2=$(nix-build $(nix-instantiate dependencies.nix)!out --no-out-link)
-[[ $outPath = $outPath2 ]]
-
-outPath2=$(nix-store -r $(nix-instantiate --add-root $TEST_ROOT/indirect dependencies.nix)!out)
-[[ $outPath = $outPath2 ]]
-
-# The order of the paths on stdout must correspond to the -A options
-# https://github.com/NixOS/nix/issues/4197
-
-input0="$(nix-build nix-build-examples.nix -A input0 --no-out-link)"
-input1="$(nix-build nix-build-examples.nix -A input1 --no-out-link)"
-input2="$(nix-build nix-build-examples.nix -A input2 --no-out-link)"
-body="$(nix-build nix-build-examples.nix -A body --no-out-link)"
-
-outPathsA="$(echo $(nix-build nix-build-examples.nix -A input0 -A input1 -A input2 -A body --no-out-link))"
-[[ "$outPathsA" = "$input0 $input1 $input2 $body" ]]
-
-# test a different ordering to make sure it fails, not just in 23 out of 24 permutations
-outPathsB="$(echo $(nix-build nix-build-examples.nix -A body -A input1 -A input2 -A input0 --no-out-link))"
-[[ "$outPathsB" = "$body $input1 $input2 $input0" ]]