Arm backend: Update gemma3n tests

[xingguo01]

Updates the Gemma3n tests to follow the common test structure
Fixes for TOSA/VGF backend of Gemma3n layer tests

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18159

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❌ 1 New Failure, 1 Cancelled Job, 1 Pending, 3 Unrelated Failures

As of commit a10f8ca with merge base b836a57 ():

NEW FAILURE - The following job has failed:

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

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

• trunk / test-mcu-cortex-m-backend / 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-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  /pytorch/executorch/kernels/portable/cpu/op_convolution.cpp:175:41: error: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
• trunk / test-arm-cortex-m-size-test (bare_metal) / linux-job (gh) (trunk failure)
  /pytorch/executorch/../executorch/kernels/portable/cpu/util/kernel_ops_util.h:233:54: error: comparison of integer expressions of different signedness: 'ssize_t' {aka 'int'} and 'size_t' {aka 'unsigned int'} [-Werror=sign-compare]
• trunk / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (gh) (trunk failure)
  /pytorch/executorch/../executorch/kernels/portable/cpu/util/kernel_ops_util.h:233:54: error: comparison of integer expressions of different signedness: 'ssize_t' {aka 'int'} and 'size_t' {aka 'unsigned int'} [-Werror=sign-compare]

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