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/gc-auto.sh

@@ -0,0 +1,81 @@
+source common.sh
+
+needLocalStore "“min-free” and “max-free” are daemon options"
+
+clearStore
+
+garbage1=$(nix store add-path --name garbage1 ./nar-access.sh)
+garbage2=$(nix store add-path --name garbage2 ./nar-access.sh)
+garbage3=$(nix store add-path --name garbage3 ./nar-access.sh)
+
+ls -l $garbage3
+POSIXLY_CORRECT=1 du $garbage3
+
+fake_free=$TEST_ROOT/fake-free
+export _NIX_TEST_FREE_SPACE_FILE=$fake_free
+echo 1100 > $fake_free
+
+fifoLock=$TEST_ROOT/fifoLock
+mkfifo "$fifoLock"
+
+expr=$(cat <<EOF
+with import ./config.nix; mkDerivation {
+  name = "gc-A";
+  buildCommand = ''
+    set -x
+    [[ \$(ls \$NIX_STORE/*-garbage? | wc -l) = 3 ]]
+
+    mkdir \$out
+    echo foo > \$out/bar
+
+    # Pretend that we run out of space
+    echo 100 > ${fake_free}.tmp1
+    mv ${fake_free}.tmp1 $fake_free
+
+    # Wait for the GC to run
+    for i in {1..20}; do
+        echo ''\${i}...
+        if [[ \$(ls \$NIX_STORE/*-garbage? | wc -l) = 1 ]]; then
+            exit 0
+        fi
+        sleep 1
+    done
+    exit 1
+  '';
+}
+EOF
+)
+
+expr2=$(cat <<EOF
+with import ./config.nix; mkDerivation {
+  name = "gc-B";
+  buildCommand = ''
+    set -x
+    mkdir \$out
+    echo foo > \$out/bar
+
+    # Wait for the first build to finish
+    cat "$fifoLock"
+  '';
+}
+EOF
+)
+
+nix build --impure -v -o $TEST_ROOT/result-A -L --expr "$expr" \
+    --min-free 1000 --max-free 2000 --min-free-check-interval 1 &
+pid1=$!
+
+nix build --impure -v -o $TEST_ROOT/result-B -L --expr "$expr2" \
+    --min-free 1000 --max-free 2000 --min-free-check-interval 1 &
+pid2=$!
+
+# Once the first build is done, unblock the second one.
+# If the first build fails, we need to postpone the failure to still allow
+# the second one to finish
+wait "$pid1" || FIRSTBUILDSTATUS=$?
+echo "unlock" > $fifoLock
+( exit ${FIRSTBUILDSTATUS:-0} )
+wait "$pid2"
+
+[[ foo = $(cat $TEST_ROOT/result-A/bar) ]]
+[[ foo = $(cat $TEST_ROOT/result-B/bar) ]]