[Offload] Run unit tests as a part of check-offload (llvm#156675)

Summary:
Add a dependnecy on the unit tests on the main check-offload test suite.
This matches what the other projects do, pass `llvm-lit` to the
directory to only run the lit tests, use the `check-offload-unit` for
only the unit tests.

Author: jhuber6

offload/test/CMakeLists.txt

@@ -61,7 +61,7 @@ add_offload_testsuite(check-offload
   "Running libomptarget tests"
   ${LIBOMPTARGET_LIT_TESTSUITES}
   EXCLUDE_FROM_CHECK_ALL
-  DEPENDS llvm-offload-device-info omptarget ${OMP_DEPEND} ${LIBOMPTARGET_TESTED_PLUGINS}
+  DEPENDS llvm-offload-device-info omptarget ${OMP_DEPEND} ${LIBOMPTARGET_TESTED_PLUGINS} check-offload-unit
   ARGS ${LIBOMPTARGET_LIT_ARG_LIST})
 
 # Add liboffload unit tests - the test binary will run on all available devices