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 68c81c7375)

tests/functional/gc-concurrent.sh

@@ -0,0 +1,59 @@
+source common.sh
+
+clearStore
+
+lockFifo1=$TEST_ROOT/test1.fifo
+mkfifo "$lockFifo1"
+
+drvPath1=$(nix-instantiate gc-concurrent.nix -A test1 --argstr lockFifo "$lockFifo1")
+outPath1=$(nix-store -q $drvPath1)
+
+drvPath2=$(nix-instantiate gc-concurrent.nix -A test2)
+outPath2=$(nix-store -q $drvPath2)
+
+drvPath3=$(nix-instantiate simple.nix)
+outPath3=$(nix-store -r $drvPath3)
+
+(! test -e $outPath3.lock)
+touch $outPath3.lock
+
+rm -f "$NIX_STATE_DIR"/gcroots/foo*
+ln -s $drvPath2 "$NIX_STATE_DIR"/gcroots/foo
+ln -s $outPath3 "$NIX_STATE_DIR"/gcroots/foo2
+
+# Start build #1 in the background.  It starts immediately.
+nix-store -rvv "$drvPath1" &
+pid1=$!
+
+# Wait for the build of $drvPath1 to start
+cat $lockFifo1
+
+# Run the garbage collector while the build is running.
+nix-collect-garbage
+
+# Unlock the build of $drvPath1
+echo "" > $lockFifo1
+echo waiting for pid $pid1 to finish...
+wait $pid1
+
+# Check that the root of build #1 and its dependencies haven't been
+# deleted.  The should not be deleted by the GC because they were
+# being built during the GC.
+cat $outPath1/foobar
+cat $outPath1/input-2/bar
+
+# Check that the build build $drvPath2 succeeds.
+# It should succeed because the derivation is a GC root.
+nix-store -rvv "$drvPath2"
+cat $outPath2/foobar
+
+rm -f "$NIX_STATE_DIR"/gcroots/foo*
+
+# The collector should have deleted lock files for paths that have
+# been built previously.
+(! test -e $outPath3.lock)
+
+# If we run the collector now, it should delete outPath1/2.
+nix-collect-garbage
+(! test -e $outPath1)
+(! test -e $outPath2)