riscv: sbi: Add tests for HSM extension

Add some tests for all of the HSM extension functions. These tests
ensure that the HSM extension functions follow the behavior as described
in the SBI specification.

Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
[Made the changes requested in the last review.]
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>