add to documentation cookbook for testing (#3544)

docs/cookbooks/testing.md

@@ -73,8 +73,9 @@ SELECT
 
 ## How to Configure a Generated Test
 
-1. Make a directory for test resources named `tests/{dataset}/{table}/{test_name}/`,
-   e.g. `tests/telemetry_derived/clients_last_seen_raw_v1/test_single_day`
+Queries are tested by running the `query.sql` with test-input tables and comparing the result to an expected table.
+1. Make a directory for test resources named `tests/sql/{project}/{dataset}/{table}/{test_name}/`,
+   e.g. `tests/sql/moz-fx-data-shared-prod/telemetry_derived/clients_last_seen_raw_v1/test_single_day`
    - `table` must match a directory named like `{dataset}/{table}`, e.g.
      `telemetry_derived/clients_last_seen_v1`
    - `test_name` should start with `test_`, e.g. `test_single_day`
@@ -83,6 +84,8 @@ SELECT
 1. Add `.yaml` files for input tables, e.g. `clients_daily_v6.yaml`
    - Include the dataset prefix if it's set in the tested query,
      e.g. `analysis.clients_last_seen_v1.yaml`
+   - Include the project prefix if it's set in the tested query,
+     e.g. `moz-fx-other-data.new_dataset.table_1.yaml`
      - This will result in the dataset prefix being removed from the query,
        e.g. `query = query.replace("analysis.clients_last_seen_v1", "clients_last_seen_v1")`
 1. Add `.sql` files for input view queries, e.g. `main_summary_v4.sql`
@@ -97,9 +100,12 @@ SELECT
      using `.isoformat()`
    - Columns named `generated_time` are removed from the result before
      comparing to `expect` because they should not be static
+   - `NULL` values should be omitted in `expect.yaml`. If a column is expected to be `NULL` don't add it to `expect.yaml`.
+     (Be careful with spreading previous rows (`-<<: *base`) here)
 1. Optionally add `.schema.json` files for input table schemas to the table directory, e.g.
-   `tests/telemetry_derived/clients_last_seen_raw_v1/clients_daily_v6.schema.json`.
+   `tests/sql/moz-fx-data-shared-prod/telemetry_derived/clients_last_seen_raw_v1/clients_daily_v6.schema.json`.
    These tables will be available for every test in the suite.
+   The `schema.json` file need to match the table name in the `query.sql` file. If it has project and dataset listed there, the schema file also needs project and dataset.
 1. Optionally add `query_params.yaml` to define query parameters
    - `query_params` must be a list