Find CppUnit2Gtest before searching for CppUnit

Important remarks:

* Assertions in helper functions do not always "bubble up" to the test
  layer, so tests would still fail but not at the line of the `ASSERT`,
  see https://google.github.io/googletest/advanced.html#assertion-placement
* Exiting Assertions are not allowed in constructors or destructors, see
  https://google.github.io/googletest/faq.html#CtorVsSetUp

Author: OlekRaymond

cmake/modules/TestTarget.cmake

@@ -16,7 +16,16 @@ endif ()
 if (NOT META_NO_CPP_UNIT)
     # allow disabling CppUnit-based tests completely
     option(ENABLE_CPP_UNIT "whether CppUnit-based tests should be enabled" ON)
-    if (NOT ENABLE_CPP_UNIT)
+    option(ENABLE_CPPUNIT2GTEST "whether to allow CppUnit2Gtest to be used instead of cppunit" ON)
+    
+    # if user has Ray's adapter layer use it instead
+    find_package(CppUnit2Gtest QUIET)
+    if(ENABLE_CPPUNIT2GTEST AND CppUnit2Gtest_FOUND)
+        find_package(GTest REQUIRED)
+        set(DETECTED_CPP_UNIT_LIB CppUnit2Gtest::CppUnit2Gtest)
+        message(STATUS "Using CppUnit2Gtest instead of cppunit")
+        # otherwise use normal cppunit
+    elseif (NOT ENABLE_CPP_UNIT)
         set(META_HAVE_TESTS NO)
         set(TEST_CONFIG_DONE YES)
         return()