The Placeholder_test should be replaced with real unit testing

[jcfreeman2]

i.e., we should improve on line 61, the auto-generated skeleton unit test code created by create_dunedaq_package:

daq_add_unit_test(Placeholder_test LINK_LIBRARIES ${PROJECT_NAME})  # Placeholder_test should be replaced with real unit tests

Along with the benefits of unit testing, making this change will ensure that this can build in a work area even if other packages have this issue. Otherwise you'll see this error:

  target with the same name already exists.  The existing target is an
  executable created in source directory
  "/home/nfs/jcfree/daqbuild_feb27/sourcecode/hermesmodules".  See
  documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  crtmodules/CMakeLists.txt:49 (daq_add_unit_test)