fix(tests): don't surpress stderr output

This is a diabolical anti-pattern and is hiding errors which exist
currently in unittests. Also we want to see _where_ in the process
stderr was emitted, stashing it away at the end erases important
context.

cmake/RunTests.cmake

@@ -78,7 +78,6 @@ execute_process(
     ${TEST_PATH}
   TIMEOUT $ENV{TEST_TIMEOUT}
   WORKING_DIRECTORY ${WORKING_DIR}
-  ERROR_VARIABLE err
   RESULT_VARIABLE res
   ${EXTRA_ARGS})
 
@@ -87,11 +86,6 @@ file(REMOVE_RECURSE ${RM_FILES})
 
 if(res)
   message(STATUS "Tests exited non-zero: ${res}")
-  if("${err}" STREQUAL "")
-    message(STATUS "No output to stderr.")
-  else()
-    message(STATUS "Output to stderr:\n${err}")
-  endif()
 
   # Dump the logfile on CI (if not displayed and moved already).
   if(CI_BUILD)