tests: add integration test for cifs /home

[zyga]

This test is a simpler rendition of the existing nfs test (nfs-support). Here I have focused only on checking the per-user home directory mount, as that captures our expectation of how the feature is to be used in practice.

[zyga]

Thank you for the reviews, I'll update this shortly.

[zyga]

This failed on:

+ tests.session -u test-remote exec snap run test-snapd-sh.with-home-plug -c 'test -h $SNAP_USER_COMMON/../current'
runuser: user test-remote does not exist or the user entry does not contain all the required fields
failed

This ... makes no sense? What's going on. I need to debug this.