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/build-remote.sh

@@ -0,0 +1,84 @@
+requireSandboxSupport
+[[ $busybox =~ busybox ]] || skipTest "no busybox"
+
+# Avoid store dir being inside sandbox build-dir
+unset NIX_STORE_DIR
+unset NIX_STATE_DIR
+
+function join_by { local d=$1; shift; echo -n "$1"; shift; printf "%s" "${@/#/$d}"; }
+
+EXTRA_SYSTEM_FEATURES=()
+if [[ -n "${CONTENT_ADDRESSED-}" ]]; then
+    EXTRA_SYSTEM_FEATURES=("ca-derivations")
+fi
+
+builders=(
+  # system-features will automatically be added to the outer URL, but not inner
+  # remote-store URL.
+  "ssh://localhost?remote-store=$TEST_ROOT/machine1?system-features=$(join_by "%20" foo ${EXTRA_SYSTEM_FEATURES[@]}) - - 1 1 $(join_by "," foo ${EXTRA_SYSTEM_FEATURES[@]})"
+  "$TEST_ROOT/machine2 - - 1 1 $(join_by "," bar ${EXTRA_SYSTEM_FEATURES[@]})"
+  "ssh-ng://localhost?remote-store=$TEST_ROOT/machine3?system-features=$(join_by "%20" baz ${EXTRA_SYSTEM_FEATURES[@]}) - - 1 1 $(join_by "," baz ${EXTRA_SYSTEM_FEATURES[@]})"
+)
+
+chmod -R +w $TEST_ROOT/machine* || true
+rm -rf $TEST_ROOT/machine* || true
+
+# Note: ssh://localhost bypasses ssh, directly invoking nix-store as a
+# child process. This allows us to test LegacySSHStore::buildDerivation().
+# ssh-ng://... likewise allows us to test RemoteStore::buildDerivation().
+nix build -L -v -f $file -o $TEST_ROOT/result --max-jobs 0 \
+  --arg busybox $busybox \
+  --store $TEST_ROOT/machine0 \
+  --builders "$(join_by '; ' "${builders[@]}")"
+
+outPath=$(readlink -f $TEST_ROOT/result)
+
+grep 'FOO BAR BAZ' $TEST_ROOT/machine0/$outPath
+
+testPrintOutPath=$(nix build -L -v -f $file --no-link --print-out-paths --max-jobs 0 \
+  --arg busybox $busybox \
+  --store $TEST_ROOT/machine0 \
+  --builders "$(join_by '; ' "${builders[@]}")"
+)
+
+[[ $testPrintOutPath =~ store.*build-remote ]]
+
+# Ensure that input1 was built on store1 due to the required feature.
+output=$(nix path-info --store $TEST_ROOT/machine1 --all)
+echo "$output" | grepQuiet builder-build-remote-input-1.sh
+echo "$output" | grepQuietInverse builder-build-remote-input-2.sh
+echo "$output" | grepQuietInverse builder-build-remote-input-3.sh
+unset output
+
+# Ensure that input2 was built on store2 due to the required feature.
+output=$(nix path-info --store $TEST_ROOT/machine2 --all)
+echo "$output" | grepQuietInverse builder-build-remote-input-1.sh
+echo "$output" | grepQuiet builder-build-remote-input-2.sh
+echo "$output" | grepQuietInverse builder-build-remote-input-3.sh
+unset output
+
+# Ensure that input3 was built on store3 due to the required feature.
+output=$(nix path-info --store $TEST_ROOT/machine3 --all)
+echo "$output" | grepQuietInverse builder-build-remote-input-1.sh
+echo "$output" | grepQuietInverse builder-build-remote-input-2.sh
+echo "$output" | grepQuiet builder-build-remote-input-3.sh
+unset output
+
+
+for i in input1 input3; do
+nix log --store $TEST_ROOT/machine0 --file "$file" --arg busybox $busybox passthru."$i" | grep hi-$i
+done
+
+# Behavior of keep-failed
+out="$(nix-build 2>&1 failing.nix \
+  --no-out-link \
+  --builders "$(join_by '; ' "${builders[@]}")"  \
+  --keep-failed \
+  --store $TEST_ROOT/machine0 \
+  -j0 \
+  --arg busybox $busybox)" || true
+
+[[ "$out" =~ .*"note: keeping build directory".* ]]
+
+build_dir="$(grep "note: keeping build" <<< "$out" | sed -E "s/^(.*)note: keeping build directory '(.*)'(.*)$/\2/")"
+[[ "foo" = $(<"$build_dir"/bar) ]]