parallel-tests: more uses for '$(am__make_dryrun)'

parallel-tests: more uses for '$(am__make_dryrun)'

With this change we refactor some recipes of the parallel-tests
harness to make them use, where appropriate, the new internal
variable '$(am__make_dryrun)' (introduced in the 'maint' branch)
when they need to determine whether make is running in "dry mode",
i.e., with "make -n".  This allows for some simplifications and
for improved uniformity.

* lib/am/check.am (check-TESTS recheck): Use $(am__make_dryrun) and
simplify accordingly.
(am--redo-logs): Delete this internal helper recipe, merged into ...
($(TEST_SUITE_LOG)): ... this, with the help of $(am__make_dryrun).
recipe accordingly.