tests/periph_rtt_min: adapt test to slow rtt
tests/periph_rtt_min: adapt test to slow rtt

pyterm only echoes after a \n is received, so use socat instead
so '.' are printed as they are generated. This will make the test
timeout for slow rtts like kinetis.

For these slow paltforms also reduce the sample size.

jasonatran/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OlegHahm/RIOT