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/misc.sh

@@ -0,0 +1,32 @@
+source common.sh
+
+# Tests miscellaneous commands.
+
+# Do all commands have help?
+#nix-env --help | grepQuiet install
+#nix-store --help | grepQuiet realise
+#nix-instantiate --help | grepQuiet eval
+#nix-hash --help | grepQuiet base32
+
+# Can we ask for the version number?
+nix-env --version | grep "$version"
+
+# Usage errors.
+expect 1 nix-env --foo 2>&1 | grep "no operation"
+expect 1 nix-env -q --foo 2>&1 | grep "unknown flag"
+
+# Eval Errors.
+eval_arg_res=$(nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 || true)
+echo $eval_arg_res | grep "at «string»:1:15:"
+echo $eval_arg_res | grep "infinite recursion encountered"
+
+eval_stdin_res=$(echo 'let a = {} // a; in a.foo' | nix-instantiate --eval -E - 2>&1 || true)
+echo $eval_stdin_res | grep "at «stdin»:1:15:"
+echo $eval_stdin_res | grep "infinite recursion encountered"
+
+# Attribute path errors
+expectStderr 1 nix-instantiate --eval -E '{}' -A '"x' | grepQuiet "missing closing quote in selection path"
+expectStderr 1 nix-instantiate --eval -E '[]' -A 'x' | grepQuiet "should be a set"
+expectStderr 1 nix-instantiate --eval -E '{}' -A '1' | grepQuiet "should be a list"
+expectStderr 1 nix-instantiate --eval -E '{}' -A '.' | grepQuiet "empty attribute name"
+expectStderr 1 nix-instantiate --eval -E '[]' -A '1' | grepQuiet "out of range"