tests: arch: arm_custom_interrupt: exclude imx943

Exclude imx943,
  - multi level interrupts feature is enabled defaultly,
    so irqsteer1 driver is enabled, then the test case
    report the build issue:
    multiple definition of z_soc_irq_enable,
    tests/arch/arm/arm_custom_interrupt/src/arm_custom_interrupt.c:47
    first defined here.

Signed-off-by: Biwen Li <biwen.li@nxp.com>