test: tweak test_fanotify() to monitor ONLY 1 file

[SteveLauC]

What does this PR do

Tweak test_fanotify() to make it monitor ONLY 1 file rather than the whole mount point.

For more info, see this comment.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

I have tested this locally, though I still think we should merge #2269 first to see CI's reaction to this change:

$ git status
On branch test_fanotify_monitor_only_one_file
Your branch is up to date with 'origin/test_fanotify_monitor_only_one_file'.

nothing to commit, working tree clean

$ sudo --preserve-env=HOME $(which cargo) test sys::test_fanotify::test_fanotify --all-features
    Finished test [unoptimized + debuginfo] target(s) in 0.03s
     Running unittests src/lib.rs (target/debug/deps/nix-df3c717de9b67128)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 19 filtered out; finished in 0.00s

     Running test/test.rs (target/debug/deps/test-64b011f0c61ee152)

running 1 test
test sys::test_fanotify::test_fanotify ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 351 filtered out; finished in 0.00s

[SteveLauC]

Merge this to fix the partial CI error of #2269