Simplify test file writes by removing unnecessary newlines

The child process tests only check that files are growing in size,
not their actual content. Removing the newlines simplifies the code
and eliminates a potential source of cross-platform issues.

This change:
- Removes \n from all test file writes
- Keeps the timestamp data for file size growth verification
- Reduces string escaping complexity

Reported-by: fweinberger

Author: felixweinberger

tests/client/test_stdio.py

@@ -389,7 +389,7 @@ async def test_stdio_client_child_process_cleanup():
             import time
             with open({repr(marker_file)}, 'a') as f:
                 while True:
-                    f.write(f"{{time.time()}}\\n")
+                    f.write(f"{time.time()}")
                     f.flush()
                     time.sleep(0.1)
             '''
@@ -493,7 +493,7 @@ async def test_stdio_client_nested_process_tree():
             grandchild_script = """import time
             with open({repr(grandchild_file)}, 'a') as f:
                 while True:
-                    f.write(f"gc {{time.time()}}\\n")
+                    f.write(f"gc {time.time()}")
                     f.flush()
                     time.sleep(0.1)"""
 
@@ -503,7 +503,7 @@ async def test_stdio_client_nested_process_tree():
             # Child writes to its file
             with open({repr(child_file)}, 'a') as f:
                 while True:
-                    f.write(f"c {{time.time()}}\\n")
+                    f.write(f"c {time.time()}")
                     f.flush()
                     time.sleep(0.1)'''
 
@@ -513,7 +513,7 @@ async def test_stdio_client_nested_process_tree():
             # Parent writes to its file
             with open({repr(parent_file)}, 'a') as f:
                 while True:
-                    f.write(f"p {{time.time()}}\\n")
+                    f.write(f"p {time.time()}")
                     f.flush()
                     time.sleep(0.1)
             """
@@ -591,7 +591,7 @@ async def test_stdio_client_early_parent_exit():
             child_script = f'''import time
             with open({repr(marker_file)}, 'a') as f:
                 while True:
-                    f.write(f"child {{time.time()}}\\n")
+                    f.write(f"child {time.time()}")
                     f.flush()
                     time.sleep(0.1)'''