test: aggregate-results.sh: consistent style. zero forks.

test: aggregate-results.sh: consistent style. zero forks.

- all variables in $((...)) without leading $
- all comparisons use -gt, -eq or -ne
- no -a nor -o inside [ ... ] expressions
- all indentation levels using one tab

Dropped unnecessary empty string check when reading results files.

Replaced pluralize() which was executed in subshell with
pluralize_s(). pluralize_s sets $s to 's' or '' based on value of
$1. Calls to pluralize_s are done in context of current shell, so
no forks to subshells executed.