Lower the shard count for sparse_bcoo_bcsr_test on TPU as well.

There are flaky timeouts in CI, and we've already lowered the shard count on multiple other platforms.

PiperOrigin-RevId: 678367575

Author: hawkinsp

tests/BUILD

@@ -997,6 +997,7 @@ jax_multiplatform_test(
         "cpu": ["--jax_num_generated_cases=40"],
         "cpu_x32": ["--jax_num_generated_cases=40"],
         "gpu": ["--jax_num_generated_cases=40"],
+        "tpu": ["--jax_num_generated_cases=40"],
     },
     shard_count = {
         "cpu": 50,