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/ca/build.sh

@@ -0,0 +1,67 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+drv=$(nix-instantiate ./content-addressed.nix -A rootCA --arg seed 1)^out
+nix derivation show "$drv" --arg seed 1
+
+buildAttr () {
+    local derivationPath=$1
+    local seedValue=$2
+    shift; shift
+    local args=("./content-addressed.nix" "-A" "$derivationPath" --arg seed "$seedValue" "--no-out-link")
+    args+=("$@")
+    nix-build "${args[@]}"
+}
+
+testDeterministicCA () {
+    [[ $(buildAttr rootCA 1) = $(buildAttr rootCA 2) ]]
+}
+
+testCutoffFor () {
+    local out1 out2
+    out1=$(buildAttr $1 1)
+    # The seed only changes the root derivation, and not it's output, so the
+    # dependent derivations should only need to be built once.
+    buildAttr rootCA 2
+    out2=$(buildAttr $1 2 -j0)
+    test "$out1" == "$out2"
+}
+
+testCutoff () {
+    # Don't directly build dependentCA, that way we'll make sure we don't rely on
+    # dependent derivations always being already built.
+    #testDerivation dependentCA
+    testCutoffFor transitivelyDependentCA
+    testCutoffFor dependentNonCA
+    testCutoffFor dependentFixedOutput
+}
+
+testGC () {
+    nix-instantiate ./content-addressed.nix -A rootCA --arg seed 5
+    nix-collect-garbage --option keep-derivations true
+    clearStore
+    buildAttr rootCA 1 --out-link $TEST_ROOT/rootCA
+    nix-collect-garbage
+    buildAttr rootCA 1 -j0
+}
+
+testNixCommand () {
+    clearStore
+    nix build --file ./content-addressed.nix --no-link
+}
+
+# Regression test for https://github.com/NixOS/nix/issues/4775
+testNormalization () {
+    clearStore
+    outPath=$(buildAttr rootCA 1)
+    test "$(stat -c %Y $outPath)" -eq 1
+}
+
+clearStore
+testNormalization
+testDeterministicCA
+clearStore
+testCutoff
+testGC
+testNixCommand