net: TestSendfilePipe failures

[gopherbot]

#!watchflakes
default <- pkg == "net" && test == "TestSendfilePipe"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestSendfilePipe
=== PAUSE TestSendfilePipe
=== CONT  TestSendfilePipe
    sendfile_test.go:388: Read did not time out
--- FAIL: TestSendfilePipe (0.08s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "net" && test == "TestSendfilePipe"

2025-12-11 17:35 gotip-linux-amd64-nosizespecializedmalloc go@9de64687 net.TestSendfilePipe (log)

=== RUN   TestSendfilePipe
=== PAUSE TestSendfilePipe
=== CONT  TestSendfilePipe
    sendfile_test.go:388: Read did not time out
--- FAIL: TestSendfilePipe (0.08s)

— watchflakes

[ruhil6789]

I can investigate whether this is a timing-sensitive assertion in TestSendfilePipe and propose a more robust test if needed.