Init runtime PAL in RunnerTest (forward fix for D108707577)

[Gasoonjia]

Summary:
Forward fix for D108707577 (llm_runner: plumb prefill temperature, #20244).

The new test RunnerTest.TextTokenGeneratorRejectsTemperatureOutOfRange aborts:

[ RUN ] RunnerTest.TextTokenGeneratorRejectsTemperatureOutOfRange
ExecuTorch PAL must be initialized before call to et_pal_current_ticks()
*** Signal 6 (SIGABRT) ***

The new test drives the temperature-rejection path, which emits an ET_LOG (and
thus calls et_pal_current_ticks()) before any model load. The RunnerTest fixture
never initializes the ExecuTorch runtime, so the timer call aborts. The valid-
temperature tests in the same fixture pass because their happy path does not log.

Fix: initialize the runtime in RunnerTest::SetUp(), matching the established
pattern used by the sibling tests in this same directory (test_text_prefiller,
test_util, test_wav_loader all call executorch::runtime::runtime_init() in
SetUp()). Applied to both the fbcode and xplat copies.

Differential Revision: D108831322

[pytorch-bot]

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❌ 1 New Failure, 1 Cancelled Job, 2 Unrelated Failures

As of commit 3cc44f5 with merge base 218cc45 ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_vit_model

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-nxp-neutron / linux-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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[meta-codesync]

@Gasoonjia has exported this pull request. If you are a Meta employee, you can view the originating Diff in D108831322.

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