test(interrupts): use custom handler for interrupt

SP registers a custom handler that checks and updates a flag. Once
the watchdog interrupt triggers, the custom handler is invoked at
the tail end of interrupt handling. This helps the test to ensure
the virtual interrupt corresponding to the trusted watchdog timer
was correctly handled.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I7e388076165d097ec7f2bb493b83fe11d2bc0f34
1 file changed