Bug #39003 mtr's diff_files command failed in pushbuild without printing a result diff

diff was actually called but result never outputted before exiting
Added extra code to dump output *unless* failure was expected

client/mysqltest.cc

@@ -3440,12 +3440,17 @@ void do_diff_files(struct st_command *command)
     die("command \"diff_files\" failed, file '%s' does not exist",
         ds_filename2.str);
 
-  if ((error= compare_files(ds_filename.str, ds_filename2.str)))
+  if ((error= compare_files(ds_filename.str, ds_filename2.str)) &&
+      match_expected_error(command, error, NULL) < 0)
   {
     /* Compare of the two files failed, append them to output
-       so the failure can be analyzed
+       so the failure can be analyzed, but only if it was not
+       expected to fail.
     */
     show_diff(&ds_res, ds_filename.str, ds_filename2.str);
+    log_file.write(&ds_res);
+    log_file.flush();
+    dynstr_set(&ds_res, 0);
   }
 
   dynstr_free(&ds_filename);

mysql-test/t/mysqltest.test

@@ -1710,10 +1710,6 @@ EOF
 --diff_files $MYSQLTEST_VARDIR/tmp/diff1.tmp $MYSQLTEST_VARDIR/tmp/diff2.tmp
 --diff_files $MYSQLTEST_VARDIR/tmp/diff2.tmp $MYSQLTEST_VARDIR/tmp/diff1.tmp
 
-# Write the below commands to a intermediary file and execute them with
-# mysqltest in --exec, since the output will vary depending on what "diff"
-# is available it is sent to /dev/null
---write_file $MYSQLTEST_VARDIR/tmp/diff.test
 # Compare files that differ in size
 --error 2
 --diff_files $MYSQLTEST_VARDIR/tmp/diff1.tmp $MYSQLTEST_VARDIR/tmp/diff3.tmp
@@ -1725,13 +1721,6 @@ EOF
 --diff_files $MYSQLTEST_VARDIR/tmp/diff1.tmp $MYSQLTEST_VARDIR/tmp/diff4.tmp
 --error 1
 --diff_files $MYSQLTEST_VARDIR/tmp/diff4.tmp $MYSQLTEST_VARDIR/tmp/diff1.tmp
-exit;
-EOF
-
-# Execute the above diffs, and send their output to /dev/null - only
-# interesting to see that it returns correct error codes
---exec $MYSQL_TEST < $MYSQLTEST_VARDIR/tmp/diff.test > /dev/null 2>&1
-
 
 # Compare equal files, again...
 --diff_files $MYSQLTEST_VARDIR/tmp/diff1.tmp $MYSQLTEST_VARDIR/tmp/diff2.tmp
@@ -1740,7 +1729,6 @@ EOF
 --remove_file $MYSQLTEST_VARDIR/tmp/diff2.tmp
 --remove_file $MYSQLTEST_VARDIR/tmp/diff3.tmp
 --remove_file $MYSQLTEST_VARDIR/tmp/diff4.tmp
---remove_file $MYSQLTEST_VARDIR/tmp/diff.test
 
 
 # ----------------------------------------------------------------------------