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/function-trace.sh

@@ -0,0 +1,67 @@
+source common.sh
+
+set +x
+
+expect_trace() {
+    expr="$1"
+    expect="$2"
+    actual=$(
+        nix-instantiate \
+            --trace-function-calls \
+            --expr "$expr" 2>&1 \
+            | grep "function-trace" \
+            | sed -e 's/ [0-9]*$//' \
+            || true
+    )
+
+    echo -n "Tracing expression '$expr'"
+    msg=$(diff -swB \
+               <(echo "$expect") \
+               <(echo "$actual")
+    ) && result=0 || result=$?
+    if [ $result -eq 0 ]; then
+        echo " ok."
+    else
+        echo " failed. difference:"
+        echo "$msg"
+        return $result
+    fi
+}
+
+# failure inside a tryEval
+expect_trace 'builtins.tryEval (throw "example")' "
+function-trace entered «string»:1:1 at
+function-trace entered «string»:1:19 at
+function-trace exited «string»:1:19 at
+function-trace exited «string»:1:1 at
+"
+
+# Missing argument to a formal function
+expect_trace '({ x }: x) { }' "
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
+"
+
+# Too many arguments to a formal function
+expect_trace '({ x }: x) { x = "x"; y = "y"; }' "
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
+"
+
+# Not enough arguments to a lambda
+expect_trace '(x: y: x + y) 1' "
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
+"
+
+# Too many arguments to a lambda
+expect_trace '(x: x) 1 2' "
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
+"
+
+# Not a function
+expect_trace '1 2' "
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
+"