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 68c81c7375)

tests/functional/dyn-drv/eval-outputOf.sh

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+# Without the dynamic-derivations XP feature, we don't have the builtin.
+nix --experimental-features 'nix-command' eval --impure  --expr \
+    'assert ! (builtins ? outputOf); ""'
+
+# Test that a string is required.
+#
+# We currently require a string to be passed, rather than a derivation
+# object that could be coerced to a string. We might liberalise this in
+# the future so it does work, but there are some design questions to
+# resolve first. Adding a test so we don't liberalise it by accident.
+expectStderr 1 nix --experimental-features 'nix-command dynamic-derivations' eval --impure --expr \
+    'builtins.outputOf (import ../dependencies.nix {}) "out"' \
+    | grepQuiet "value is a set while a string was expected"
+
+# Test that "DrvDeep" string contexts are not supported at this time
+#
+# Like the above, this is a restriction we could relax later.
+expectStderr 1 nix --experimental-features 'nix-command dynamic-derivations' eval --impure --expr \
+    'builtins.outputOf (import ../dependencies.nix {}).drvPath "out"' \
+    | grepQuiet "has a context which refers to a complete source and binary closure. This is not supported at this time"
+
+# Test using `builtins.outputOf` with static derivations
+testStaticHello () {
+    nix eval --impure --expr \
+        'with (import ./text-hashed-output.nix); let
+           a = hello.outPath;
+           b = builtins.outputOf (builtins.unsafeDiscardOutputDependency hello.drvPath) "out";
+         in builtins.trace a
+           (builtins.trace b
+             (assert a == b; null))'
+}
+
+# Test with a regular old input-addresed derivation
+#
+# `builtins.outputOf` works without ca-derivations and doesn't create a
+# placeholder but just returns the output path.
+testStaticHello
+
+# Test with content addressed derivation.
+NIX_TESTS_CA_BY_DEFAULT=1 testStaticHello
+
+# Test with derivation-producing derivation
+#
+# This is hardly different from the preceding cases, except that we're
+# only taking 1 outputOf out of 2 possible outputOfs. Note that
+# `.outPath` could be defined as `outputOf drvPath`, which is what we're
+# testing here. The other `outputOf` that we're not testing here is the
+# use of _dynamic_ derivations.
+nix eval --impure --expr \
+    'with (import ./text-hashed-output.nix); let
+       a = producingDrv.outPath;
+       b = builtins.outputOf (builtins.builtins.unsafeDiscardOutputDependency producingDrv.drvPath) "out";
+     in builtins.trace a
+       (builtins.trace b
+         (assert a == b; null))'
+
+# Test with unbuilt output of derivation-producing derivation.
+#
+# This function similar to `testStaticHello` used above, but instead of
+# checking the property on a constant derivation, we check it on a
+# derivation that's from another derivation's output (outPath).
+testDynamicHello () {
+    nix eval --impure --expr \
+        'with (import ./text-hashed-output.nix); let
+           a = builtins.outputOf producingDrv.outPath "out";
+           b = builtins.outputOf (builtins.outputOf (builtins.unsafeDiscardOutputDependency producingDrv.drvPath) "out") "out";
+         in builtins.trace a
+           (builtins.trace b
+             (assert a == b; null))'
+}
+
+# inner dynamic derivation is input-addressed
+testDynamicHello
+
+# inner dynamic derivation is content-addressed
+NIX_TESTS_CA_BY_DEFAULT=1 testDynamicHello