test(tail): fix follow name truncate failing on macos

[eduardomourar]

Fixes: #12335

test_follow_name_truncate1 used fixed sleeps between spawning tail and truncating/restoring the watched file, which raced against a slow-starting child process on macOS CI and could miss both events entirely.

Replaced the pre-mutation delay with a wait for tail's initial output, which is only printed after the file watcher is registered — making the test deterministic instead of probabilistic.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 153.09 MB (+904 KB, +0.58%)

Significant per-binary changes:
  expr     2.76 MB ->    3.65 MB  (+908 KB, +32.11%)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)