testdir/runnvim.sh: create messages file always (#11503)

src/nvim/testdir/runnvim.sh

@@ -82,6 +82,11 @@ main() {(
   fi
   if test "$FAILED" = 1 ; then
     echo "Test $test_name failed, see output above and summary for more details" >> test.log
+    # When Neovim crashed/aborted it might not have created messages.
+    # test.log itself is used as an indicator to exit non-zero in the Makefile.
+    if ! test -f message; then
+      cp -a test.log messages
+    fi
   fi
 )}