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, 10 insertions, 0 deletions

diff --git a/tests/functional/lang/eval-okay-search-path.nix b/tests/functional/lang/eval-okay-search-path.nix
new file mode 100644
index 000000000..6fe33decc
--- /dev/null
+++ b/tests/functional/lang/eval-okay-search-path.nix
@@ -0,0 +1,10 @@
+with import ./lib.nix;
+with builtins;
+
+assert isFunction (import <nix/fetchurl.nix>);
+
+assert length __nixPath == 5;
+assert length (filter (x: baseNameOf x.path == "dir4") __nixPath) == 1;
+
+import <a.nix> + import <b.nix> + import <c.nix> + import <dir5/c.nix>
+  + (let __nixPath = [ { path = ./dir2; } { path = ./dir1; } ]; in import <a.nix>)