admission: segment test files by central type

Pure code movement change. Carve out
{io_load_listener,tokens_linear_model}_test.go using the same non-test
partitioning scheme as earlier commits. Within granter_test.go, move the
supporting types further below in the file.

Release note: None

pkg/util/admission/BUILD.bazel

@@ -40,7 +40,9 @@ go_test(
     srcs = [
         "disk_bandwidth_test.go",
         "granter_test.go",
+        "io_load_listener_test.go",
         "store_token_estimation_test.go",
+        "tokens_linear_model_test.go",
         "work_queue_test.go",
     ],
     data = glob(["testdata/**"]),