src/sage/doctest/forker.py: add long times

Fix the CI warnings,

  2025-07-14T18:33:42.3293571Z ##[warning]slow doctest:
  2025-07-14T18:33:42.3294863Z     W.join()  # Wait for worker to finish
  2025-07-14T18:33:42.3295367Z Test ran for 12.99s cpu, 16.77s wall

  2025-07-14T18:33:58.2926836Z ##[warning]slow doctest:
  2025-07-14T18:33:58.2928161Z     W.join()  # Wait for worker to finish
  2025-07-14T18:33:58.2928647Z Test ran for 13.04s cpu, 15.61s wall

  2025-07-14T18:34:13.9622114Z ##[warning]slow doctest:
  2025-07-14T18:34:13.9635400Z     W.join()
  2025-07-14T18:34:13.9639916Z Test ran for 12.89s cpu, 0.03s wall

  2025-07-14T18:34:29.4681581Z ##[warning]slow doctest:
  2025-07-14T18:34:29.4684229Z     W.join()
  2025-07-14T18:34:29.4684558Z Test ran for 12.97s cpu, 15.48s wall

by adding "# long time" to these tests.

Author: orlitzky

src/sage/doctest/forker.py

@@ -2184,6 +2184,7 @@ class should be accessed by the child process.
 
     EXAMPLES::
 
+        sage: # long time
         sage: from sage.doctest.forker import DocTestWorker, DocTestTask
         sage: from sage.doctest.sources import FileDocTestSource
         sage: from sage.doctest.reporting import DocTestReporter
@@ -2308,6 +2309,7 @@ def start(self):
 
         TESTS::
 
+            sage: # long time
             sage: from sage.doctest.forker import DocTestWorker, DocTestTask
             sage: from sage.doctest.sources import FileDocTestSource
             sage: from sage.doctest.reporting import DocTestReporter
@@ -2347,6 +2349,7 @@ def read_messages(self):
 
         EXAMPLES::
 
+            sage: # long time
             sage: from sage.doctest.forker import DocTestWorker, DocTestTask
             sage: from sage.doctest.sources import FileDocTestSource
             sage: from sage.doctest.reporting import DocTestReporter
@@ -2381,6 +2384,7 @@ def save_result_output(self):
 
         EXAMPLES::
 
+            sage: # long time
             sage: from sage.doctest.forker import DocTestWorker, DocTestTask
             sage: from sage.doctest.sources import FileDocTestSource
             sage: from sage.doctest.reporting import DocTestReporter