Fix warnings in frontend tests

[tylerflex]

The front end tests trigger the following warnings in scene and mode solver

tests/test_components/test_scene.py::test_plot_1d_scene
tests/test_components/test_simulation.py::test_plot_1d_sim
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/components/scene.py:478: UserWarning: Attempting to set identical low and high xlims makes transformation singular; automatically expanding.
    ax.set_xlim(hlim)

tests/test_plugins/test_mode_solver.py::test_mode_solver_nan_pol_fraction
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/plugins/mode/solver.py:464: RuntimeWarning: divide by zero encountered in divide
    Hx = h_field[:N, :] / (1j * neff - keff)

tests/test_plugins/test_mode_solver.py::test_mode_solver_nan_pol_fraction
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/plugins/mode/solver.py:464: RuntimeWarning: invalid value encountered in divide
    Hx = h_field[:N, :] / (1j * neff - keff)

tests/test_plugins/test_mode_solver.py::test_mode_solver_nan_pol_fraction
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/plugins/mode/solver.py:465: RuntimeWarning: divide by zero encountered in divide
    Hy = h_field[N:, :] / (1j * neff - keff)

tests/test_plugins/test_mode_solver.py::test_mode_solver_nan_pol_fraction
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/plugins/mode/solver.py:465: RuntimeWarning: invalid value encountered in divide
    Hy = h_field[N:, :] / (1j * neff - keff)

tests/test_plugins/test_mode_solver.py::test_mode_solver_nan_pol_fraction
  /Users/twhughes/Documents/Flexcompute/tidy3d-docs/tidy3d/tidy3d/plugins/mode/solver.py:474: RuntimeWarning: invalid value encountered in multiply
    H *= -1j / ETA_0

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

it would be good to fix these before this list grows