Tests: Teach RunCMake to report command-line on failure

When a test sets RunCMake_TEST_COMMAND, report the command line
on failure to make it easier to run by hand for debugging.

Tests/RunCMake/RunCMake.cmake

@@ -93,6 +93,10 @@ function(run_cmake test)
   if(RunCMake_TEST_FAILED)
     set(msg "${RunCMake_TEST_FAILED}\n${msg}")
   endif()
+  if(msg AND RunCMake_TEST_COMMAND)
+    string(REPLACE ";" "\" \"" command "\"${RunCMake_TEST_COMMAND}\"")
+    set(msg "${msg}Command was:\n command> ${command}\n")
+  endif()
   if(msg)
     string(REGEX REPLACE "\n" "\n actual-out> " actual_out " actual-out> ${actual_stdout}")
     string(REGEX REPLACE "\n" "\n actual-err> " actual_err " actual-err> ${actual_stderr}")