Makefile.am: Let make bats use ${TESTCASES} too

To limit the test cases that run when running make bats, use
${TESTCASES} instead of passing BATS a directory.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
t/test_dh_exec.bats: Make it work in the source directory too

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>