tests: kernel: gen_isr_table: Disable RISC-V direct ISR test

This commit disables the RISC-V direct ISR test due to the broken
`vectors` section placement with the IRQ vector table enabled,
introduced by the commit d2f8ec70235208f4a70e371ccb1ed8dfe0f573c5.

For more details, refer to the GitHub issue #49903 tracking this bug.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>