Otherwise, CTEST_USE_LAUNCHERS can be set to true in a driving ctest
script, and everything looks like it would work and reports 0 build errors,
but actually there can be any amount of errors and they are just not reported.
By adding this check cmake now errors out if CTEST_USE_LAUNCHERS is enabled, but
the RULE properties are not set, which would lead to a not-working ctest run.

Alex