test: cover prometheus_mp telemetry dir hardening (#2113)

## What?

Three gtest cases for the `prometheus_mp` telemetry-directory hardening
in `resolveMultiprocDir()`: a directory the exporter creates is 0700,
and an existing group-writable or world-writable one warns and still
serves. Tests only, no production change.

## Why?

Deferred from the diff freeze on #1920 (NIX-1683). None of it could be
observed before: the mp fixture hands the exporter a directory it
already created at 0700, so the warning can never fire and the
production `chmod` is indistinguishable from the fixture's own. Dropping
the `created` guard, or reordering `permissions` before
`create_directories`, broke nothing visible.

The foreign-owner branch (`st_uid != geteuid()`) is left uncovered by
design, not by oversight — see below.

## How?

All three tests point `NIXL_TELEMETRY_MULTIPROC_DIR` at a
**subdirectory** of the fixture's directory. That child path is free to
be absent or loose without any fixture surgery, and the fixture still
needs its own 0700 chmod to stop a permissive CI umask from reddening
the suite.

<details>
<summary>Mutation testing, the two loose cases, and the uncovered
branch</summary>

Each test was verified against the mutation it exists to catch, and
confirmed to fail on that one alone:

| Mutation | Result |
|---|---|
| Drop the `permissions(..., owner_all, replace)` call |
`CreatedTelemetryDirIsPrivate` fails: mode is the umask default 0755 |
| Drop the `created` guard (chmod unconditionally) |
`WorldWritable...`/`GroupWritable...` fail: the loose directory is
silently fixed before `stat()`, so the warning count falls to 0 |
| Narrow the mask to `S_IWGRP` alone | `WorldWritableTelemetryDirWarns`
fails; the group case still warns |
| Narrow the mask to `S_IWOTH` alone | `GroupWritableTelemetryDirWarns`
fails; the world case still warns |

The last two are why the loose case is 0720 and 0702 rather than a
single 0777: 0777 sets both bits, so it cannot tell the halves of the
mask apart. Either single-bit case subsumes 0777. They are two `TEST_F`s
rather than a `TEST_P` because the fixture derives its directory from
the suite and test name, and an instantiated parameterised suite puts a
`/` in both, nesting that directory and leaving the intermediate level
behind `TearDown`.

`CreatedTelemetryDirIsPrivate` also asserts the subdirectory is absent
first: otherwise `create_directories()` reports `created == false`, the
`chmod` never runs, and the 0700 assertion is satisfied by whatever left
the directory behind — which a failed `TearDown` under `--gtest_repeat`
could do, since every iteration reuses the same path.

All mutations were reverted. Verified with `--gtest_repeat=5`: 93 tests
per iteration, all passing, no un-ignored warnings.

**Foreign owner.** Covering it needs `chown`, so the test would run only
as root and skip on every unprivileged lane — an assertion whose meaning
depends on the environment, which is the shape NIX-1710 just removed
from the tracing suite, and which NIX-1623 is trying to make fatal. The
prerequisite is a second unprivileged uid in the test container; that is
CI infrastructure and belongs in its own change.

</details>

---------

Signed-off-by: Efraim Eygin <eeygin@nvidia.com>