Skip parallel tests on machines having a single CPU

Otherwise this is what happens:

The following tests FAILED:
        5762 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.spmc_sum (Failed)
        5763 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpsc_sum (Failed)
        5764 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpmc_sum (Failed)
        5767 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.spmc_dynamicsize (Failed)
        5768 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.spmc_fixedsize (Failed)
        5769 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpsc_dynamicsize (Failed)
        5770 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpsc_fixedsize (Failed)
        5771 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpmc_dynamicsize (Failed)
        5772 - contrib/parallel/buffer_queue_parallel_test::buffer_queue.mpmc_fixedsize (Failed)
Errors while running CTest

test/unit/contrib/parallel/CMakeLists.txt

@@ -2,5 +2,9 @@
 # SPDX-FileCopyrightText: 2016-2024 Knut Reinert & MPI für molekulare Genetik
 # SPDX-License-Identifier: CC0-1.0
 
-seqan3_test (buffer_queue_parallel_test.cpp)
+include(ProcessorCount)
+ProcessorCount(NPROC)
+if(NPROC GREATER 1)
+  seqan3_test (buffer_queue_parallel_test.cpp)
+endif()
 seqan3_test (buffer_queue_test.cpp)