Allow ssp_test:read to pass more reliably

It appears that the stackframe layout can be slightly different depending on
compiler and target architecture. For example, when using CHERI LLVM for RISC-V
we can actually overflow the buffer by up to 8 bytes without SSP detecting it.
Fix this by increasing the overflow to 15 bytes.

Reviewed By: ngie, emaste
Differential Revision: https://reviews.freebsd.org/D28997