tests: Split utils in 2 subtests

utils test is a little too large for hammer's small RO, so we split
it in 2 test: utils and utils_str. Instead of one test that requires
about 8kb extra flash, we have 2 tests that take respectively
3.4kb (utils_str) and 4.6kb (utils) of extra flash.

BRANCH=none
BUG=chromium:726113
TEST=make BOARD=hammer tests -j
     util/flash_ec --board=hammer --image=build/hammer/test-utils.bin
     runtest => pass
     Repeat with test-utils_str.bin
TEST=Before this change:
       make runtests -j
       ./util/run_host_test utils | grep Running | sort > old
     Apply this change:
       make runtests -j
       (./util/run_host_test utils; ./util/run_host_test utils_str) \
               | grep Running | sort > new
       diff old new => No difference (except timing)

Change-Id: I917d572e671d6ce0a8799508761f55de7bd83133
Reviewed-on: https://chromium-review.googlesource.com/514604
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
4 files changed