split testsuite/main and qtest/test_runner rules for faster qtest-only testing
split testsuite/main and qtest/test_runner rules for faster qtest-only testing

The makefile used to bundle compilation of testsuite/main and
qtest/test_runner. Splitting them allow to compile qtest/test_runner
only, which roughly halves the overall testing time of 'hot compiles'
(most object files already cached). To test a specific module use:
  TESTABLE="src/batFoo.ml" make qtest-byte

This is a slight improvement over thelema's work on [issue #138]. On
my machine, testing only batFloat.ml dropped from 2.60s to 1.30s.
 [issue #138]: https://github.com/ocaml-batteries-team/batteries-included/issues/138

bennn/batteries-included