Use generic pkill target in evergreen.yml to kill all unit test processes

etc/evergreen.yml lists a few unit test processes as pkill targets. We should just terminate all unit test processes with a more generic pkill target (i.e., "_test") to avoid having to add processes as we see build failures.

A recent build failure with a test that's been running since June:

 [2015/08/06 19:42:04.174] mci            77670 131.9  0.0  2451864    280   ??  S    25Jun15 53089:06.58 build/opt/mongo/db/repl/repl_coordinator_impl_elect_v1_test

– https://evergreen.mongodb.com/task/mongodb_mongo_master_osx_108_compile_1b5b55cb48ff9daca74177b86d336613d207c5d1_15_08_06_21_27_09#/log/ALL