parallel-tests: avoid issue with overly long lines in sed input

parallel-tests: avoid issue with overly long lines in sed input

See automake bug#10437:
  <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10437>
and coreutils bug#10427:
  <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10427#8>

* lib/am/check.am (recheck, recheck-html): In order to strip
trailing whitespace from the definition of the `$list' variable,
we used to invoke sed in a way that could cause it to get passed
overly long input lines, causing spurious failures.  So rework
the logic of the recipe to avoid any sed invocation, relying on
simpler shell idioms instead.
(check-TESTS): Reorganize the recipe to be more similar to the
one of `recheck', for consistency and simplicity.
* NEWS: Update.

Report and analysis by Paul Eggert.