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, 43 deletions

diff --git a/tests/lang/eval-okay-versions.nix b/tests/lang/eval-okay-versions.nix
deleted file mode 100644
index e9111f5f4..000000000
--- a/tests/lang/eval-okay-versions.nix
+++ /dev/null
@@ -1,43 +0,0 @@
-let
-
-  name1 = "hello-1.0.2";
-  name2 = "hello";
-  name3 = "915resolution-0.5.2";
-  name4 = "xf86-video-i810-1.7.4";
-  name5 = "name-that-ends-with-dash--1.0";
-
-  eq = 0;
-  lt = builtins.sub 0 1;
-  gt = 1;
-
-  versionTest = v1: v2: expected:
-    let d1 = builtins.compareVersions v1 v2;
-        d2 = builtins.compareVersions v2 v1;
-    in d1 == builtins.sub 0 d2 && d1 == expected;
-
-  tests = [
-    ((builtins.parseDrvName name1).name == "hello")
-    ((builtins.parseDrvName name1).version == "1.0.2")
-    ((builtins.parseDrvName name2).name == "hello")
-    ((builtins.parseDrvName name2).version == "")
-    ((builtins.parseDrvName name3).name == "915resolution")
-    ((builtins.parseDrvName name3).version == "0.5.2")
-    ((builtins.parseDrvName name4).name == "xf86-video-i810")
-    ((builtins.parseDrvName name4).version == "1.7.4")
-    ((builtins.parseDrvName name5).name == "name-that-ends-with-dash")
-    ((builtins.parseDrvName name5).version == "-1.0")
-    (versionTest "1.0" "2.3" lt)
-    (versionTest "2.1" "2.3" lt)
-    (versionTest "2.3" "2.3" eq)
-    (versionTest "2.5" "2.3" gt)
-    (versionTest "3.1" "2.3" gt)
-    (versionTest "2.3.1" "2.3" gt)
-    (versionTest "2.3.1" "2.3a" gt)
-    (versionTest "2.3pre1" "2.3" lt)
-    (versionTest "2.3pre3" "2.3pre12" lt)
-    (versionTest "2.3a" "2.3c" lt)
-    (versionTest "2.3pre1" "2.3c" lt)
-    (versionTest "2.3pre1" "2.3q" lt)
-  ];
-
-in (import ./lib.nix).and tests