fn: fix TestWriteFile when running as root

[starius]

Change Description

Root can bypass read-only file permissions, which made the test failure confusing: a file was overwritten even though it shouldn't have been. This is caused by process capability CAP_DAC_OVERRIDE which root processes often have in Linux. To workaround this, the owner of the file is changed to uid=1000 and gid=1000 (root can do it). If a file belongs to a world-writable sticky dir (like /tmp where the tested file is created) and fs.protected_regular kernel feature is enabled (a common practice on modern Linux distros), such file can't be overwritten by root.

Steps to Test

$ cd fn/

$ go test -v -run TestWriteFile
=== RUN   TestWriteFile
=== PAUSE TestWriteFile
=== RUN   TestWriteFileRemove
=== PAUSE TestWriteFileRemove
=== CONT  TestWriteFile
=== CONT  TestWriteFileRemove
--- PASS: TestWriteFileRemove (0.03s)
--- PASS: TestWriteFile (0.04s)
PASS
ok      github.com/lightningnetwork/lnd/fn/v2   0.040s

$ sudo su
# go test -v -run TestWriteFile
=== RUN   TestWriteFile
=== PAUSE TestWriteFile
=== RUN   TestWriteFileRemove
=== PAUSE TestWriteFileRemove
=== CONT  TestWriteFile
=== CONT  TestWriteFileRemove
--- PASS: TestWriteFile (0.01s)
--- PASS: TestWriteFileRemove (0.01s)
PASS
ok      github.com/lightningnetwork/lnd/fn/v2   0.017s
# 

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