CTestCoverageCollectGCOV: Capture gcov output in log file

The output of gcov can be large and we only display it in a failure
case.  Put it in a log file to avoid accumulating large output in
memory.

Issue: #20554

Modules/CTestCoverageCollectGCOV.cmake

@@ -139,7 +139,8 @@ function(ctest_coverage_collect_gcov)
   endif()
   execute_process(COMMAND
     ${gcov_command} ${GCOV_GCOV_OPTIONS} ${gcda_files}
-    OUTPUT_VARIABLE out
+    OUTPUT_FILE "${coverage_dir}/gcov.log"
+    ERROR_FILE  "${coverage_dir}/gcov.log"
     RESULT_VARIABLE res
     WORKING_DIRECTORY ${coverage_dir})
 
@@ -149,7 +150,7 @@ function(ctest_coverage_collect_gcov)
 
   if(NOT "${res}" EQUAL 0)
     if (NOT GCOV_QUIET)
-      message(STATUS "Error running gcov: ${res} ${out}")
+      message(STATUS "Error running gcov: ${res}, see\n  ${coverage_dir}/gcov.log")
     endif()
   endif()
   # create json file with project information