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, 18 insertions, 0 deletions

diff --git a/tests/functional/pass-as-file.sh b/tests/functional/pass-as-file.sh
new file mode 100644
index 000000000..2c0bc5031
--- /dev/null
+++ b/tests/functional/pass-as-file.sh
@@ -0,0 +1,18 @@
+source common.sh
+
+clearStore
+
+outPath=$(nix-build --no-out-link -E "
+with import ./config.nix;
+
+mkDerivation {
+  name = \"pass-as-file\";
+  passAsFile = [ \"foo\" ];
+  foo = [ \"xyzzy\" ];
+  builder = builtins.toFile \"builder.sh\" ''
+    [ \"\$(basename \$fooPath)\" = .attr-1bp7cri8hplaz6hbz0v4f0nl44rl84q1sg25kgwqzipzd1mv89ic ]
+    [ \"\$(cat \$fooPath)\" = xyzzy ]
+    touch \$out
+  '';
+}
+")