testsuite: coverage: allow gcov data dump directly on console

When there is no buffer for gcov hex data,
dump them directly to console.
This saves RAM which can be used to hold more gcov data.

Reduce number of gcov hex data send by console -
do not send space between every two digits.

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>