test: extend homed test to test home dir "maximization"

test: extend homed test to test home dir "maximization"

This moves the backing store to a separate tmpfs which we can nicely put
a size limit on to make sure we can test maximization sanely: if we ask
for the home dir to be grown really large it should effectively only be
grown until the size of the backing tmpfs.

(While we are at it, also set a cheaper KDF so that we don't waste CI
cycles for password hashing that aren#t secure anyway.)