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, 86 insertions, 0 deletions

diff --git a/tests/functional/experimental-features.sh b/tests/functional/experimental-features.sh
new file mode 100644
index 000000000..607bf0a8e
--- /dev/null
+++ b/tests/functional/experimental-features.sh
@@ -0,0 +1,86 @@
+source common.sh
+
+# Skipping these two for now, because we actually *do* want flags and
+# config settings to always show up in the manual, just be marked
+# experimental. Will reenable once the manual generation takes advantage
+# of the JSON metadata on this.
+#
+# # Without flakes, flake options should not show up
+# # With flakes, flake options should show up
+#
+# function grep_both_ways {
+#     nix --experimental-features 'nix-command' "$@" | grepQuietInverse flake
+#     nix --experimental-features 'nix-command flakes' "$@" | grepQuiet flake
+#
+#     # Also, the order should not matter
+#     nix "$@" --experimental-features 'nix-command' | grepQuietInverse flake
+#     nix "$@" --experimental-features 'nix-command flakes' | grepQuiet flake
+# }
+#
+# # Simple case, the configuration effects the running command
+# grep_both_ways show-config
+#
+# # Medium case, the configuration effects --help
+# grep_both_ways store gc --help
+
+# Test settings that are gated on experimental features; the setting is ignored
+# with a warning if the experimental feature is not enabled. The order of the
+# `setting = value` lines in the configuration should not matter.
+
+# 'flakes' experimental-feature is disabled before, ignore and warn
+NIX_CONFIG='
+  experimental-features = nix-command
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is disabled after, ignore and warn
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled before, process
+NIX_CONFIG='
+  experimental-features = nix-command flakes
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled after, process
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command flakes
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+function exit_code_both_ways {
+    expect 1 nix --experimental-features 'nix-command' "$@" 1>/dev/null
+    nix --experimental-features 'nix-command flakes' "$@" 1>/dev/null
+
+    # Also, the order should not matter
+    expect 1 nix "$@" --experimental-features 'nix-command' 1>/dev/null
+    nix "$@" --experimental-features 'nix-command flakes' 1>/dev/null
+}
+
+exit_code_both_ways show-config --flake-registry 'https://no'
+
+# Double check these are stable
+nix --experimental-features '' --help 1>/dev/null
+nix --experimental-features '' doctor --help 1>/dev/null
+nix --experimental-features '' repl --help 1>/dev/null
+nix --experimental-features '' upgrade-nix --help 1>/dev/null
+
+# These 3 arguments are currently given to all commands, which is wrong (as not
+# all care). To deal with fixing later, we simply make them require the
+# nix-command experimental features --- it so happens that the commands we wish
+# stabilizing to do not need them anyways.
+for arg in '--print-build-logs' '--offline' '--refresh'; do
+    nix --experimental-features 'nix-command' "$arg" --help 1>/dev/null
+    expect 1 nix --experimental-features '' "$arg" --help 1>/dev/null
+done