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-cache.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+# The substituters didn't work prior to this time.
+requireDaemonNewerThan "2.18.0pre20230808"
+
+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[@]}"
+}
+
+copyAttr () {
+    local derivationPath=$1
+    local seedValue=$2
+    shift; shift
+    local args=("-f" "./content-addressed.nix" "$derivationPath" --arg seed "$seedValue")
+    args+=("$@")
+    # Note: to copy CA derivations, we need to copy the realisations, which
+    # currently requires naming the installables, not just the derivation output
+    # path.
+    nix copy --to file://$cacheDir "${args[@]}"
+}
+
+testRemoteCacheFor () {
+    local derivationPath=$1
+    clearCache
+    copyAttr "$derivationPath" 1
+    clearStore
+    # Check nothing gets built.
+    buildAttr "$derivationPath" 1 --option substituters file://$cacheDir --no-require-sigs |& grepQuietInverse " will be built:"
+}
+
+testRemoteCache () {
+    testRemoteCacheFor rootCA
+    testRemoteCacheFor dependentCA
+    testRemoteCacheFor dependentNonCA
+    testRemoteCacheFor dependentFixedOutput
+    testRemoteCacheFor dependentForBuildCA
+    testRemoteCacheFor dependentForBuildNonCA
+}
+
+clearStore
+testRemoteCache