In general tests should be parallizable since they interact only with a
deterministic set of batches. The exception is list-batches, especially
with pagination, so run that one sequentially.
This doesn't make much difference for the current set of tests, but in
the near future we will have tests that create batches, which take tens
of seconds to even start running.
Rearrange subtests to be primarily organized by tool, which is more
understandable and easier to filter with `-run`. Test helper methods can
still be called multiple times in different subtests for different
tools.
Sample test output showing the new structure:
```
--- PASS: TestServerlessSparkToolEndpoints (2.01s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches (1.78s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success (1.23s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/filtered (0.34s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/empty (0.40s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/omit_page_size (0.42s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/one_page (0.43s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/20_batches (0.44s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/success/two_pages (0.54s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/errors (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/errors/negative_page_size (0.01s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/errors/zero_page_size (0.01s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/auth (0.77s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/auth/no_auth_token (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/auth/invalid_auth_token (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/list-batches/auth/valid_auth_token (0.18s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch (0.09s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/errors (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/errors/full_batch_name (0.01s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/errors/missing_batch (0.11s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/success (0.21s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/success/found_batch (0.11s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/auth (0.60s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/auth/invalid_auth_token (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/auth/no_auth_token (0.00s)
--- PASS: TestServerlessSparkToolEndpoints/parallel-tool-tests/get-batch/auth/valid_auth_token (0.11s)
```
