[ng] check: reduce code duplication in some auxiliary variables

[ng] check: reduce code duplication in some auxiliary variables

* lib/am/parallel-tests.am (am.test-suite.awk-trs-loop/BEGIN,
am.test-suite.awk-trs-loop/END, am.test-suite.awk-functions): New, hold
fragments of awk scripts extracted from (and common to) ...
(am__create_global_log, am__count_test_result): ... these variables.
Some related improvements to comments, and minor reformatting.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>