pw_cpu_exception_armv7m: Unit test updates
pw_cpu_exception_armv7m: Unit test updates

 - Adds a test that ensures nested faults are correctly handled.
 - Properly clears CFSR and HFSR registers when handling exceptions.
 - Fixes a typo useage -> usage.

Verified test passes on the stm32f429i-disc1 target.

Change-Id: Ib00f5d9f5ff59bba101b451415f6bd75076d4863

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