tests: arm: sw_vector_table: fix build on many non systick platforms

In 4c93fcd3, the default use case has been changed to support setting
a custom handler, but it's written in a way to only support MCUX and
SYSTICK based platforms. For any other platform the build break because
of undefined TIMER_IRQ_HANDLER and TIMER_IRQ_NUM.

Fix the conditional so that the custom timer line is only entered if a
custom handler is defined. The test would probably stil not run on those
platforms until a custom case is defined pointing at the custom timer
interrput handler, but at least it won't break CI.

Also drop a closing endif comment as that clearly became misleading.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>