util/tests: Reduce load from sparse-array test

Only create 8 threads rather than 16, since otherwise that can hammer
fork a bit too much. Since it creates a bunch of threads, don't try to
run it in parallel with other tests either.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31059>

src/util/tests/sparse_array_test.cpp

@@ -53,7 +53,7 @@ TEST(SparseArrayTest, Multithread)
       struct util_sparse_array arr;
       util_sparse_array_init(&arr, sizeof(uint32_t), node_size);
 
-      thrd_t threads[16];
+      thrd_t threads[8];
       for (unsigned i = 0; i < ARRAY_SIZE(threads); i++) {
          int ret = thrd_create(&threads[i], test_thread, &arr);
          ASSERT_EQ(ret, thrd_success);