Do not measure with a per-thread CPU clock under the M:N scheduler

assert_linear_performance() picks CLOCK_THREAD_CPUTIME_ID when it is
available.  That clock belongs to the native thread that reads it, and an
M:N thread moves between native threads, so the two reads around one
measurement can come from different native threads and the elapsed time
comes out negative:

    Expected 0 to be <= -2.2150775009999997.

Reproduced with RUBY_MN_THREADS=2, 1500 fork/waitpid pairs per run: the
Ruby thread changed native thread 1, 9 and 10 times over three runs, and
the clock went backwards 1, 5 and 6 times, by as much as 0.38s.  The
default, =1 and =-1 never moved the main thread and never went backwards.
CLOCK_PROCESS_CPUTIME_ID stayed monotonic across 15 such migrations.

Prefer process CPU time when the M:N scheduler is on.  This only changes
which clock the assertion uses; it does not relax the assertion.

ruby/ruby carries the same change as 0e06087548.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: ko1

lib/core_assertions.rb

@@ -873,10 +873,15 @@ def assert_all_assertions_foreach(msg = nil, *keys, &block)
       end
       alias all_assertions_foreach assert_all_assertions_foreach
 
-      %w[
+      clocks = %w[
         CLOCK_THREAD_CPUTIME_ID CLOCK_PROCESS_CPUTIME_ID
         CLOCK_MONOTONIC
-      ].find do |c|
+      ]
+      # CLOCK_THREAD_CPUTIME_ID is per native thread, and an M:N thread moves
+      # between native threads, so it can go backwards and yield a negative
+      # elapsed time.  Measure process CPU time there instead.
+      clocks.shift if RUBY_DESCRIPTION.include?("+MN")
+      clocks.find do |c|
         if Process.const_defined?(c)
           [c.to_sym, Process.const_get(c)].find do |clk|
             begin