test(internal): make comm-propagation/filter tests hermetic

TestCommPropagation and TestEventTypeFiltering/FdEventFiltering were
flaky in full `mage test` runs (passing in isolation). For synthetic
events whose tid was not in the comm cache, the event loop fell back to
commResolver's default resolveFn, which reads /proc/<tid>/comm on the
host. The fixed test pids/tids are small (e.g. defaultTid+100 == 111,
defaultPid+1 == 11) and collide with real transient kernel threads
(e.g. kworker/0:1-events), so the resolved comm depended on what
happened to be running on the host at that instant.

Fix: use commResolver's existing injectable resolveFn seam. Add a
newHermeticCommResolver() test helper whose resolveFn returns ("", nil)
and never touches /proc, and inject it into TestCommPropagation (via
eventLoopConfig.commResolver) and newEventLoopWithFilter (used by
TestEventTypeFiltering). No production code changes.

Assertions are unchanged: positive comm names still come from the
synthetic OpenEvent.Comm bytes; cache-miss tids now deterministically
resolve to empty regardless of host state. Updated the stale
"use a very large TID to avoid /proc collisions" comment accordingly.

Verified: -count=50 (affected tests) and -race -count=10 green, full
`mage test` and `mage build` green.

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

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