[ng] check: refactoring to reduce code duplication

[ng] check: refactoring to reduce code duplication

* lib/am/check.am (am__strip_suffixes): New internal function, factoring
the code used to "normalize" a list of test scripts (i.e., removing any
registered test extension and any leading $(srcdir) component).
(am__TEST_BASES, am__xfail_test_bases): Redefine using that function.
(am__cooked_tests, am__cooked_xfail_tests): Remove as obsolete.
* t/parallel-tests-empty-tests.sh: Don't bother checking the contents
of $(am__cooked_tests) anymore.

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