test: add test database cleaner in subprocess (#19844)

fixes https://github.com/coder/internal/issues/927

Adds a small subprocess that outlives the testing process to clean up any leaked test databases.

coderd/database/dbtestutil/cleanercmd/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "github.com/coder/coder/v2/coderd/database/dbtestutil"
+
+func main() {
+	dbtestutil.RunCleaner()
+}