test: add coverage for setitimer (signal-safe) and statfs/fstatfs

setitimer/getitimer (di0): no scenario previously exercised the classic
interval-timer family. Add intervalTimerNoop, which calls
setitimer(ITIMER_REAL, &{0,0,0,0}, NULL) with an all-zero itimerval so the
timer is disarmed and NO SIGALRM is ever scheduled (mirrors miscAlarmCancel's
alarm(0) and posixTimerLifecycle's never-firing pattern), followed by a safe
getitimer read. Both are KindNull on enter / UNCLASSIFIED on exit, so
TestIntervalTimerNoop asserts enter_setitimer and enter_getitimer presence.

statfs/fstatfs (7j0): stat_test.go covered stat/fstat/lstat/newfstatat/statx
but not the statfs family. Add statStatfs, which calls syscall.Statfs(path)
(enter_statfs path_event captures the pathname) and syscall.Fstatfs(fd)
(enter_fstatfs fd_event). TestStatStatfs asserts enter_statfs PathContains the
filename and enter_fstatfs presence. Covers audits it (fstatfs) and e00 (statfs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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