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/gc-concurrent.sh

@@ -1,59 +0,0 @@
-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)