Skip multicast test if there is no python3 on VM

For example RHEL 8 image don't have Python installed by default,
so this test will fail when it should be skipped.

With this patch it will be skipped as before trying to start
multicast sender/receiver scripts, there is simple check if
python3 binary exists on VM.

Change-Id: I25cb5b159330ac80777f8ea784e98fa06e6e056f
Closes-Bug: #1842934
1 file changed