test: hsr: Move common code to hsr_common.sh file

Some of the code already present in the hsr_ping.sh test program can be
moved to a separate script file, so it can be reused by other HSR
functionality (like HSR-SAN) tests.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>