overlay: test concurrent copy up

overlay: test concurrent copy up

Run 4 process pairs, each competing over copy up of 1K files
in 1 directory. One opponent touches all files in the directory
and the other truncates all files in the directory.

This test does NOT check for concurrent copy up support. It only
fails on unexpected errors of any of the touch/truncate operations.

The test full output should demonstrate the expected results -
for kernel with concurrent copy up support, truncate workers are
not delayed by touch workers. As a result, truncate workers will
finish their work much sooner than a test run without concurrent
copy up support.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>