doc: re-running tests

Author: steve-chavez

docs/adding-tests.md

@@ -13,16 +13,33 @@ These are super easy: simply add `.sql` files to the
 [tests/smoke](./../tests/smoke/) directory, then:
 
 ```
-nix flake check
+nix flake check -L
 ```
 
+(`-L` prints logs to stderrr, for more details see `man nix`)
+
 These files are run using `pg_prove`; they pretty much behave exactly like how
 you expect; you can read
 [the pgTAP documentation](https://pgtap.org/documentation.html) for more.
 
 For a good example of a pgTAP test as a pull request, check out
 [pull request #4](https://github.com/supabase/nix-postgres/pull/4/files).
 
+## Re-running tests
+
+`nix flake check` gets its results cached, so if you do it again the tests won't rerun. If you change a file then it will run again.
+
+If you want to force rerun without modifying a file, you can do:
+
+```
+nix build .#checks.x86_64-linux.psql_14 --rebuild
+nix build .#checks.x86_64-linux.psql_15 --rebuild
+```
+
+Limitation: currently there's no way to rerun all the tests, so you have to specify the check attribute.
+
+To get the correct attribute (`#checks.x86_64-linux.psql_15` above), you can do `nix flake show`. This will show a tree with all the output attributes.
+
 ## Migration tests
 
 > **NOTE**: Currently, migration tests _do not happen in CI_. They can only be