gh-47798: Fix flaky pipeline tests where downstream exits without reading

In tests where the second command exits immediately (sys.exit(N))
without reading stdin, the first command's stdout flush during
interpreter shutdown can hit a readerless pipe and yield exit code
120, breaking assertions on returncodes[0] == 0.  Seen on Windows
free-threading and FreeBSD refleak buildbots.  The first command's
output is unused in these tests; switch it to "pass".

Author: gpshead

Lib/test/test_subprocess.py

@@ -2150,7 +2150,7 @@ def test_pipeline_error_check(self):
         """Test that check=True raises PipelineError on failure"""
         with self.assertRaises(subprocess.PipelineError) as cm:
             subprocess.run_pipeline(
-                [sys.executable, "-c", 'print("hello")'],
+                [sys.executable, "-c", "pass"],
                 [sys.executable, "-c", "import sys; sys.exit(1)"],
                 capture_output=True, check=True
             )
@@ -2261,7 +2261,7 @@ def test_pipeline_completed_repr(self):
     def test_pipeline_check_returncodes_method(self):
         """Test CompletedPipeline.check_returncodes() method"""
         result = subprocess.run_pipeline(
-            [sys.executable, "-c", 'print("hello")'],
+            [sys.executable, "-c", "pass"],
             [sys.executable, "-c", "import sys; sys.exit(5)"],
             capture_output=True
         )
@@ -2996,7 +2996,10 @@ def test_command_in_pipeline_error(self):
             stderr=subprocess.DEVNULL)
         try:
             subprocess.run_pipeline(
-                [sys.executable, "-c", 'print("x")'],
+                # cmd0 must not write to stdout: cmd1 exits without
+                # reading, and a flush to a readerless pipe during
+                # interpreter shutdown can yield exit code 120.
+                [sys.executable, "-c", "pass"],
                 explicit,
                 capture_output=True, check=True,
             )