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, 29 insertions, 0 deletions

diff --git a/tests/functional/why-depends.sh b/tests/functional/why-depends.sh
new file mode 100644
index 000000000..9680bf80e
--- /dev/null
+++ b/tests/functional/why-depends.sh
@@ -0,0 +1,29 @@
+source common.sh
+
+clearStore
+
+cp ./dependencies.nix ./dependencies.builder0.sh ./config.nix $TEST_HOME
+
+cd $TEST_HOME
+
+nix why-depends --derivation --file ./dependencies.nix input2_drv input1_drv
+nix why-depends --file ./dependencies.nix input2_drv input1_drv
+
+nix-build ./dependencies.nix -A input0_drv -o dep
+nix-build ./dependencies.nix -o toplevel
+
+FAST_WHY_DEPENDS_OUTPUT=$(nix why-depends ./toplevel ./dep)
+PRECISE_WHY_DEPENDS_OUTPUT=$(nix why-depends ./toplevel ./dep --precise)
+
+# Both outputs should show that `input-2` is in the dependency chain
+echo "$FAST_WHY_DEPENDS_OUTPUT" | grepQuiet input-2
+echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grepQuiet input-2
+
+# But only the “precise” one should refer to `reference-to-input-2`
+echo "$FAST_WHY_DEPENDS_OUTPUT" | grepQuietInverse reference-to-input-2
+echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grepQuiet reference-to-input-2
+
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '2p' | grepQuiet "└───reference-to-input-2 -> "
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '3p' | grep "    →" | grepQuiet "dependencies-input-2"
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '4p' | grepQuiet "    └───input0: …"                          # in input-2, file input0
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '5p' | grep "        →" | grepQuiet "dependencies-input-0"    # is dependencies-input-0 referenced