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, 33 deletions

diff --git a/tests/gc-non-blocking.sh b/tests/gc-non-blocking.sh
deleted file mode 100644
index 0d781485d..000000000
--- a/tests/gc-non-blocking.sh
+++ /dev/null
@@ -1,33 +0,0 @@
-# Test whether the collector is non-blocking, i.e. a build can run in
-# parallel with it.
-source common.sh
-
-needLocalStore "the GC test needs a synchronisation point"
-
-clearStore
-
-fifo=$TEST_ROOT/test.fifo
-mkfifo "$fifo"
-
-dummy=$(nix store add-path ./simple.nix)
-
-running=$TEST_ROOT/running
-touch $running
-
-(_NIX_TEST_GC_SYNC=$fifo nix-store --gc -vvvvv; rm $running) &
-pid=$!
-
-sleep 2
-
-outPath=$(nix-build --max-silent-time 60 -o "$TEST_ROOT/result" -E "
-  with import ./config.nix;
-  mkDerivation {
-    name = \"non-blocking\";
-    buildCommand = \"set -x; test -e $running; mkdir \$out; echo > $fifo\";
-  }")
-
-wait $pid
-
-(! test -e $running)
-(! test -e $dummy)
-test -e $outPath