Tests/RunCMake: Add option to print custom details in failure output

Tests/RunCMake/README.rst

@@ -71,6 +71,8 @@ but do not actually build anything.  To add a test:
     Top of test binary tree
 
    and an failure must store a message in ``RunCMake_TEST_FAILED``.
+   The check script may optionally set ``RunCMake_TEST_FAILURE_MESSAGE``
+   with additional text to be included in the message if the test fails.
 
 To speed up local testing, you can choose to run only a subset of
 ``run_cmake()`` tests in a ``RunCMakeTest.cmake`` script by using the

Tests/RunCMake/RunCMake.cmake

@@ -248,6 +248,7 @@ function(run_cmake test)
     endif()
   endforeach()
   unset(RunCMake_TEST_FAILED)
+  unset(RunCMake_TEST_FAILURE_MESSAGE)
   if(RunCMake-check-file AND EXISTS ${top_src}/${RunCMake-check-file})
     include(${top_src}/${RunCMake-check-file})
   else()
@@ -278,6 +279,9 @@ function(run_cmake test)
         string(APPEND msg "Actual ${o}:\n${actual_${o}}\n")
       endif()
     endforeach()
+    if(RunCMake_TEST_FAILURE_MESSAGE)
+      string(APPEND msg "${RunCMake_TEST_FAILURE_MESSAGE}")
+    endif()
     message(SEND_ERROR "${test}${RunCMake_TEST_VARIANT_DESCRIPTION} - FAILED:\n${msg}")
   else()
     message(STATUS "${test}${RunCMake_TEST_VARIANT_DESCRIPTION} - PASSED")