Wait for snapd to seed before installing snaps
    
    Currently dist-git tests fail with the following error:
    
    	+ snap list
    	No snaps are installed yet. Try 'snap install hello-world'.
    	+ snap install test-snapd-tools
    	error: too early for operation, device not yet seeded or device model not acknowledged
    
    It looks like the test ought to wait for snapd to seed to start
    accepting installation requests.
    
    Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>