Ran the regression suites one test at a time so the timing tests stop flaking

The first CI run of this branch failed 13 tests across the five ThreadX
configurations, and every one of them measures time:

  threadx_timer_simple_test                  all five configurations
  threadx_timer_information_test             four
  threadx_timer_multiple_accuracy_test       trace_build
  threadx_time_get_set_test                  stack_checking_rand_fill_build
  threadx_thread_sleep_for_100ticks_test     trace_build
  threadx_thread_simple_sleep_non_clear_test default_build_coverage

The suites ran with CTEST_PARALLEL_LEVEL=4, so four of them shared a runner
with fewer cores than that. A test that asserts on tick counts fails when it
does not get the processor, and --repeat until-pass:2 only doubled the cost of
each one: the ThreadX job took twenty minutes against SMP's three.

The same suites pass serially, all 95 tests in all five configurations, and the
coverage evidence on this branch has been measured serially throughout. This is
also what the suites already do upstream, where they moved to serial execution
for this reason.

The cost is wall clock and nothing else, and the whole run is still well inside
the job timeout.

Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>

scripts/test_smp.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-CTEST_PARALLEL_LEVEL=4 $(dirname `realpath $0`)/../test/smp/cmake/run.sh test all
+CTEST_PARALLEL_LEVEL=1 $(dirname `realpath $0`)/../test/smp/cmake/run.sh test all

scripts/test_tx.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-CTEST_PARALLEL_LEVEL=4 $(dirname `realpath $0`)/../test/tx/cmake/run.sh test all
+CTEST_PARALLEL_LEVEL=1 $(dirname `realpath $0`)/../test/tx/cmake/run.sh test all