tests: move list of tests in its own Makefile fragment
This change will make it easier to merge maint into master, which
has a similar Makefile setup in the testsuite.
* tests/Makefile.am (include): Inclusion of ...
* tests/list-of-tests.mk: ... this new file.
* tests/Makefile.am (TESTS): Move most of its content into,
and redefine in function of ...
* tests/list-of-tests.mk (handwritten_TESTS): ... this new
variable.
* tests/gen-parallel-tests: Update.