!306 tc-testing: fix a bug in gitignore of tc-testing

Merge Pull Request from: @liujing_20201110 
 
when we modfying kernel, commit it to our environment building. we find a error
that is "tools/testing/selftests/tc-testing/plugins" failed: No such file or directory"

we find plugins directory is ignored in
"tools/testing/selftests/tc-testing/.gitignore", but the plugins directory
is need in "tools/testing/selftests/tc-testing/Makefile" 
 
Link:https://gitee.com/openeuler/kernel/pulls/306

 
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>