gh-154427: Check the access time in UtimeTests only if it is stored (GH-154428)

HAMMER2 on DragonFly BSD does not store the access time and os.stat()
returns the modification time instead.  Probe the file system, like it
is already done for the timestamp resolution.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_os/test_os.py

@@ -1058,11 +1058,18 @@ def support_subsecond(self, filename):
                 or (st.st_mtime != st[8])
                 or (st.st_ctime != st[9]))
 
+    def support_atime(self, filename):
+        # Heuristic to check if the filesystem stores the access time.
+        # Use whole seconds, to not depend on the timestamp resolution.
+        os.utime(filename, (1, 2))
+        return os.stat(filename).st_atime == 1
+
     def _test_utime(self, set_time, filename=None):
         if not filename:
             filename = self.fname
 
         support_subsecond = self.support_subsecond(filename)
+        support_atime = self.support_atime(filename)
         if support_subsecond:
             # Timestamp with a resolution of 1 microsecond (10^-6).
             #
@@ -1089,18 +1096,22 @@ def _test_utime(self, set_time, filename=None):
             # digits worth of sub-second precision.
             # Some day it would be good to fix this upstream.
             delta=1e-5
-            self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-5)
+            if support_atime:
+                self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-5)
+                self.assertAlmostEqual(st.st_atime_ns, atime_ns, delta=1e9 * 1e-5)
             self.assertAlmostEqual(st.st_mtime, mtime_ns * 1e-9, delta=1e-5)
-            self.assertAlmostEqual(st.st_atime_ns, atime_ns, delta=1e9 * 1e-5)
             self.assertAlmostEqual(st.st_mtime_ns, mtime_ns, delta=1e9 * 1e-5)
         else:
             if support_subsecond:
-                self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-6)
+                if support_atime:
+                    self.assertAlmostEqual(st.st_atime, atime_ns * 1e-9, delta=1e-6)
                 self.assertAlmostEqual(st.st_mtime, mtime_ns * 1e-9, delta=1e-6)
             else:
-                self.assertEqual(st.st_atime, atime_ns * 1e-9)
+                if support_atime:
+                    self.assertEqual(st.st_atime, atime_ns * 1e-9)
                 self.assertEqual(st.st_mtime, mtime_ns * 1e-9)
-            self.assertEqual(st.st_atime_ns, atime_ns)
+            if support_atime:
+                self.assertEqual(st.st_atime_ns, atime_ns)
             self.assertEqual(st.st_mtime_ns, mtime_ns)
 
     def test_utime(self):