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, 0 insertions, 59 deletions

diff --git a/tests/lang/eval-okay-arithmetic.nix b/tests/lang/eval-okay-arithmetic.nix
deleted file mode 100644
index 7e9e6a0b6..000000000
--- a/tests/lang/eval-okay-arithmetic.nix
+++ /dev/null
@@ -1,59 +0,0 @@
-with import ./lib.nix;
-
-let {
-
-  /* Supposedly tail recursive version:
-
-  range_ = accum: first: last:
-    if first == last then ([first] ++ accum)
-    else range_ ([first] ++ accum) (builtins.add first 1) last;
-
-  range = range_ [];
-  */
-
-  x = 12;
-
-  err = abort "urgh";
-
-  body = sum
-    [ (sum (range 1 50))
-      (123 + 456)
-      (0 + -10 + -(-11) + -x)
-      (10 - 7 - -2)
-      (10 - (6 - -1))
-      (10 - 1 + 2)
-      (3 * 4 * 5)
-      (56088 / 123 / 2)
-      (3 + 4 * const 5 0 - 6 / id 2)
-
-      (builtins.bitAnd 12 10) # 0b1100 & 0b1010 =  8
-      (builtins.bitOr  12 10) # 0b1100 | 0b1010 = 14
-      (builtins.bitXor 12 10) # 0b1100 ^ 0b1010 =  6
-
-      (if 3 < 7 then 1 else err)
-      (if 7 < 3 then err else 1)
-      (if 3 < 3 then err else 1)
-
-      (if 3 <= 7 then 1 else err)
-      (if 7 <= 3 then err else 1)
-      (if 3 <= 3 then 1 else err)
-
-      (if 3 > 7 then err else 1)
-      (if 7 > 3 then 1 else err)
-      (if 3 > 3 then err else 1)
-
-      (if 3 >= 7 then err else 1)
-      (if 7 >= 3 then 1 else err)
-      (if 3 >= 3 then 1 else err)
-
-      (if 2 > 1 == 1 < 2 then 1 else err)
-      (if 1 + 2 * 3 >= 7 then 1 else err)
-      (if 1 + 2 * 3 < 7 then err else 1)
-
-      # Not integer, but so what.
-      (if "aa" < "ab" then 1 else err)
-      (if "aa" < "aa" then err else 1)
-      (if "foo" < "foobar" then 1 else err)
-    ];
-
-}