tests: kernel: gen_isr_table: Don't use IRQ1 if already used

IRQ 1 is used by the LiteX timer, so we can't use it for the test:

gen_isr_tables.py: error: multiple registrations at table_index 1 for
irq 1 (0x1)

Existing section
['.irq.WEST_TOPDIR/zephyr/tests/kernel/gen_isr_table/src/main.c.0'],
new section .irq.WEST_TOPDIR/zephyr/drivers/timer/litex_timer.c.0

Has IRQ_CONNECT or IRQ_DIRECT_CONNECT accidentally been invoked on the
same irq multiple times?

Issue #92194

Signed-off-by: Tom Hughes <tomhughes@chromium.org>