## Summary

`PgRelcachePreloadMemMinimalTest.FreshEmptyDbStaysBounded`,
`PgRelcachePreloadMemMinimalTest.TotalPgMemoryStaysBounded`, and
`PgRelcachePreloadMemNormalTest.FreshEmptyDbStaysBounded` fail on macOS.

They read the PostgreSQL backend's memory usage from
`/proc/<pid>/status` via `PgMiniTestBase::PeakRssMb` / `ProcFileValue`.
macOS has no procfs, so the open fails and the tests error with:

```
Bad status: IO error (yb/yql/pgwrapper/pg_mini_test_base.cc:242): cannot open /proc/<pid>/status
```

These RSS/PSS memory-introspection tests are inherently Linux-only. Wrap
all three in `YB_DISABLE_TEST_ON_MACOS` so they build as `DISABLED_*` on
macOS and keep running on Linux.

Fixes #32661.

## Test plan

- [x] Confirmed via full-run and rerun logs that all three failures
share the same macOS-only cause (`pg_mini_test_base.cc:242: cannot open
/proc/<pid>/status`)
- [x] Tests build as `DISABLED_*` on macOS after the change (no longer
executed)
- [x] Tests remain enabled and unaffected on Linux (only the macOS build
path is gated)