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/
```

tests/functional/lang/eval-fail-fromTOML-timestamps.nix

@@ -0,0 +1,130 @@
+builtins.fromTOML ''
+  key = "value"
+  bare_key = "value"
+  bare-key = "value"
+  1234 = "value"
+
+  "127.0.0.1" = "value"
+  "character encoding" = "value"
+  "ʎǝʞ" = "value"
+  'key2' = "value"
+  'quoted "value"' = "value"
+
+  name = "Orange"
+
+  physical.color = "orange"
+  physical.shape = "round"
+  site."google.com" = true
+
+  # This is legal according to the spec, but cpptoml doesn't handle it.
+  #a.b.c = 1
+  #a.d = 2
+
+  str = "I'm a string. \"You can quote me\". Name\tJos\u00E9\nLocation\tSF."
+
+  int1 = +99
+  int2 = 42
+  int3 = 0
+  int4 = -17
+  int5 = 1_000
+  int6 = 5_349_221
+  int7 = 1_2_3_4_5
+
+  hex1 = 0xDEADBEEF
+  hex2 = 0xdeadbeef
+  hex3 = 0xdead_beef
+
+  oct1 = 0o01234567
+  oct2 = 0o755
+
+  bin1 = 0b11010110
+
+  flt1 = +1.0
+  flt2 = 3.1415
+  flt3 = -0.01
+  flt4 = 5e+22
+  flt5 = 1e6
+  flt6 = -2E-2
+  flt7 = 6.626e-34
+  flt8 = 9_224_617.445_991_228_313
+
+  bool1 = true
+  bool2 = false
+
+  odt1 = 1979-05-27T07:32:00Z
+  odt2 = 1979-05-27T00:32:00-07:00
+  odt3 = 1979-05-27T00:32:00.999999-07:00
+  odt4 = 1979-05-27 07:32:00Z
+  ldt1 = 1979-05-27T07:32:00
+  ldt2 = 1979-05-27T00:32:00.999999
+  ld1 = 1979-05-27
+  lt1 = 07:32:00
+  lt2 = 00:32:00.999999
+
+  arr1 = [ 1, 2, 3 ]
+  arr2 = [ "red", "yellow", "green" ]
+  arr3 = [ [ 1, 2 ], [3, 4, 5] ]
+  arr4 = [ "all", 'strings', """are the same""", ''''type'''']
+  arr5 = [ [ 1, 2 ], ["a", "b", "c"] ]
+
+  arr7 = [
+    1, 2, 3
+  ]
+
+  arr8 = [
+    1,
+    2, # this is ok
+  ]
+
+  [table-1]
+  key1 = "some string"
+  key2 = 123
+
+
+  [table-2]
+  key1 = "another string"
+  key2 = 456
+
+  [dog."tater.man"]
+  type.name = "pug"
+
+  [a.b.c]
+  [ d.e.f ]
+  [ g .  h  . i ]
+  [ j . "ʞ" . 'l' ]
+  [x.y.z.w]
+
+  name = { first = "Tom", last = "Preston-Werner" }
+  point = { x = 1, y = 2 }
+  animal = { type.name = "pug" }
+
+  [[products]]
+  name = "Hammer"
+  sku = 738594937
+
+  [[products]]
+
+  [[products]]
+  name = "Nail"
+  sku = 284758393
+  color = "gray"
+
+  [[fruit]]
+    name = "apple"
+
+    [fruit.physical]
+      color = "red"
+      shape = "round"
+
+    [[fruit.variety]]
+      name = "red delicious"
+
+    [[fruit.variety]]
+      name = "granny smith"
+
+  [[fruit]]
+    name = "banana"
+
+    [[fruit.variety]]
+      name = "plantain"
+''