Skip tests if test subdir is missing

Problem reported by Jeffrey Walton in:
https://lists.gnu.org/r/emacs-devel/2019-05/msg00041.html
* Makefile.in (CHECK_TARGETS): New macro; use it
to simplify 'check' and similar rules.
($(CHECK_TARGETS)): If tests are missing, do not fail
after issuing a diagnostic.  Just skip the tests.
1 job for master in 59 minutes and 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1637
test-all

00:59:03

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:315: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:290: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1