Fix Windows test failures due to incorrect f-string escaping

The child process tests were failing on Windows because the nested
f-strings had incorrect brace escaping. When an f-string is defined
inside another f-string, the inner f-string expressions should use
single braces to be evaluated, not double braces.

Fixed three instances where {{time.time()}} should be {time.time()}:
- Line 385: test_stdio_client_child_process_cleanup
- Line 485: test_stdio_client_nested_process_tree (grandchild)
- Line 579: test_stdio_client_early_parent_exit

This allows the time.time() expression to be properly evaluated when
the child scripts are executed, enabling the tests to verify that
child processes are writing to their marker files.

Reported-by: Felix Weinberger

Author: felixweinberger

tests/client/test_stdio.py

@@ -482,7 +482,7 @@ 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()}}")
+                    f.write(f"gc {time.time()}")
                     f.flush()
                     time.sleep(0.1)\"\"\"