build_common.am: use test_{programs|scripts|helpers} variables for tests

- instead of TESTS, check_PROGRAMS directly; logic of where test programs
  (TESTS, check_PROGRAMS), test scripts (TESTS only) and test helper
  programs (check_PROGRAMS only) go is now handled in one place
Non-recursive build: add helper Makefiles to {src|python|test}/ and subdirs

- these point to top-level non-recursive Makefile which contains build rules
- allows libraries to be re-made from within {src|python|test}/ and subdirs
LALPulsar: update .gitignore
LALSimulation: update .gitignore