Remove relative paths from run_test_suite.sh

[mnlevy1981]

While reviewing another PR, @klindsay28 pointed out that run_test_suite.sh sets ${MARBL_ROOT} but then relies on relative paths for arguments to various scripts (./MARBL_generate_settings_file.py -f ../defaults/json/settings_${shortname}.json, find ../../input_files/settings -type f, etc). Many relative paths to subdirectories of ${MARBL_ROOT}/tests/input_files/ so it might make sense to also define MARBL_INPUTDIR to reduce line length and improve readability.