tools: selftests: riscv: Add test count for vstate_prctl

Add the test count to drop the warning message.
"Planned tests != run tests (0 != 1)"

Fixes: 7cf6198c ("selftests: Test RISC-V Vector prctl interface")
Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andy Chiu <AndybnAC@gmail.com>
Link: https://lore.kernel.org/r/20241220091730.28006-3-yongxuan.wang@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>