Make timing selftest less sensitive
Make timing selftest less sensitive

- allow up to 12.5% security/error margin
- use larger delays
- this avoid the security/error margin being too low

The test used to fail about 1 out of 6 times on some buildbots VMs, but never
failed on the physical machines used for development.

ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls