test/gtest: stop the inert-tracer test depending on NIXL_PLUGIN_DIR (#2092)

## What?

`Tracing.RequestedBackendWithoutPluginIsInert` asked `makeTracer` for
the `nvtx` backend and expected a null tracer. It now asks for a backend
name no plugin can provide, matching what the sibling test
`Tracing.MakeTracerUnknownBackendReturnsNull` already did; the name
lives in one `kUnloadableBackend` constant used by both.

## Why?

The old assertion rested on a property of the *environment*, not of the
code: its own comment said "no `libtrace_backend_*.so` is registered in
this unit binary". Whenever a real `libtrace_backend_nvtx.so` is
discoverable, the plugin loads, `makeTracer` correctly returns a live
tracer, and the test fails. That is the case in the dev container, where
`NIXL_PLUGIN_DIR` points at an install tree containing the NVTX trace
plugin, and it would be the case in any CI leg that installs NIXL before
running the `unit` suite.

The test has been permanently red locally for weeks and was written off
as "pre-existing, unrelated" in the validation notes of three telemetry
PRs (#1952, #2054, #2086). Beyond the recurring explanation, a genuine
regression in the null-tracer / inert-`Span` path would have been
indistinguishable from the known-red state.

Coverage is unchanged: this test exists to prove that a null tracer
leaves call sites on the safe default-constructed `Span` path, which
never required NVTX specifically. Real NVTX behaviour stays covered by
the e2e `TestTransferTracing` tests, which load the actual plugin.

Tracking: NIX-1710.

<details>
<summary>Rejected alternative, and verification</summary>

Clearing `NIXL_PLUGIN_DIR` inside the test via the existing
`gtest::ScopedEnv` helper does not work reliably: `getPluginDir()` is
read once in the `nixlPluginManager` constructor and cached in
`plugin_dirs_` for the process lifetime, so the outcome would depend on
whether an earlier test in the binary already touched the plugin
manager.

Verified both directions with the container's default `NIXL_PLUGIN_DIR`,
i.e. with no workaround applied:

- Baseline (this change stashed, rebuilt): exactly one failure,
`RequestedBackendWithoutPluginIsInert`, reporting a live tracer pointer
where `nullptr` was expected.
- With the change: all 17 `Tracing.*` tests pass, and the full `unit`
suite is 157 passed / 2 skipped / 3 failed — the three failures being
`objCrtTestFixture.TransferBelowThreshold` and the two
`ObjClientTests/objParamTestFixture.ReadTransfer` params, which need an
object-storage endpoint this container does not provide.

</details>

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Improved tracing plugin-loading tests to consistently cover
environments where the requested backend is unavailable.
* Updated inert-tracer coverage to validate behavior when no tracing
plugin can be loaded.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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