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/db-migration.sh

@@ -1,28 +0,0 @@
-# Test that we can successfully migrate from an older db schema
-
-source common.sh
-
-# Only run this if we have an older Nix available
-# XXX: This assumes that the `daemon` package is older than the `client` one
-if [[ -z "${NIX_DAEMON_PACKAGE-}" ]]; then
-    skipTest "not using the Nix daemon"
-fi
-
-killDaemon
-
-# Fill the db using the older Nix
-PATH_WITH_NEW_NIX="$PATH"
-export PATH="${NIX_DAEMON_PACKAGE}/bin:$PATH"
-clearStore
-nix-build simple.nix --no-out-link
-nix-store --generate-binary-cache-key cache1.example.org $TEST_ROOT/sk1 $TEST_ROOT/pk1
-dependenciesOutPath=$(nix-build dependencies.nix --no-out-link --secret-key-files "$TEST_ROOT/sk1")
-fixedOutPath=$(IMPURE_VAR1=foo IMPURE_VAR2=bar nix-build fixed.nix -A good.0 --no-out-link)
-
-# Migrate to the new schema and ensure that everything's there
-export PATH="$PATH_WITH_NEW_NIX"
-info=$(nix path-info --json $dependenciesOutPath)
-[[ $info =~ '"ultimate":true' ]]
-[[ $info =~ 'cache1.example.org' ]]
-nix verify -r "$fixedOutPath"
-nix verify -r "$dependenciesOutPath" --sigs-needed 1 --trusted-public-keys $(cat $TEST_ROOT/pk1)