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, 42 insertions, 0 deletions

diff --git a/tests/functional/read-only-store.sh b/tests/functional/read-only-store.sh
new file mode 100644
index 000000000..d63920c19
--- /dev/null
+++ b/tests/functional/read-only-store.sh
@@ -0,0 +1,42 @@
+source common.sh
+
+enableFeatures "read-only-local-store"
+
+needLocalStore "cannot open store read-only when daemon has already opened it writeable"
+
+clearStore
+
+happy () {
+    # We can do a read-only query just fine with a read-only store
+    nix --store local?read-only=true path-info $dummyPath
+    
+    # We can "write" an already-present store-path a read-only store, because no IO is actually required
+    nix-store --store local?read-only=true --add dummy
+}
+## Testing read-only mode without forcing the underlying store to actually be read-only
+
+# Make sure the command fails when the store doesn't already have a database
+expectStderr 1 nix-store --store local?read-only=true --add dummy | grepQuiet "database does not exist, and cannot be created in read-only mode"
+
+# Make sure the store actually has a current-database, with at least one store object
+dummyPath=$(nix-store --add dummy)
+
+# Try again and make sure we fail when adding a item not already in the store
+expectStderr 1 nix-store --store local?read-only=true --add eval.nix | grepQuiet "attempt to write a readonly database"
+
+# Test a few operations that should work with the read-only store in its current state
+happy
+
+## Testing read-only mode with an underlying store that is actually read-only
+
+# Ensure store is actually read-only
+chmod -R -w $TEST_ROOT/store
+chmod -R -w $TEST_ROOT/var
+
+# Make sure we fail on add operations on the read-only store
+# This is only for adding files that are not *already* in the store
+expectStderr 1 nix-store --add eval.nix | grepQuiet "error: opening lock file '$(readlink -e $TEST_ROOT)/var/nix/db/big-lock'"
+expectStderr 1 nix-store --store local?read-only=true --add eval.nix | grepQuiet "Permission denied"
+
+# Test the same operations from before should again succeed
+happy