[C++] Dataset tests not running with Acero refactor

[westonpace]

Describe the bug, including details regarding any error messages, version, and platform.

The change appears to be due to a change to ADD_ARROW_DATASET_TEST:

  add_arrow_test(${REL_TEST_NAME}
                 EXTRA_LINK_LIBS
                 ${ARROW_DATASET_TEST_LINK_LIBS}
                 PREFIX
                 ${PREFIX}
                 SOURCES # This is new
                 test_util_internal.cc # This is new
                 LABELS
                 ${LABELS}
                 ${ARG_UNPARSED_ARGUMENTS})

If SOURCES is specified then the implicit ${REL_TEST_NAME}.cc is not added. As a result, all it is compiling is test_util_internal.cc and no tests are discovered.

Component(s)

C++