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.)
Since the GNU `diff` utility uses grep-style regular expressions[0], which
use the BRE style, we need to tweak the regex to make it work properly
(most notably - in BRE the meta characters need to be escaped).
```
$ diff a b
21c21
< Volume Key: 256bit
---
> Volume Key: 257bit
25c25
< Disk Ceiling: 323.2M
---
> Disk Ceiling: 323.1M
$ diff -I '^\s*Disk (Size|Free|Floor|Ceiling):' a b
21c21
< Volume Key: 256bit
---
> Volume Key: 257bit
25c25
< Disk Ceiling: 323.2M
---
> Disk Ceiling: 323.1M
$ diff -I '^\s*Disk \(Size\|Free\|Floor\|Ceiling\):' a b && echo OK
21c21
< Volume Key: 256bit
---
> Volume Key: 257bit
```
Caught in one of the nightly CentOS CI cron jobs.
[0] https://www.gnu.org/software/diffutils/manual/html_node/Specified-Lines.html
Forcing a specific fs for the image is usually a bad idea because the initrd
(borrowed from the host) is likely to include only support for the filesystem
used by the host's rootfs.
Since the point of this test is to check aliases on mount units, there's no
specific need for ext4, hence drop any parts that request or rely on ext4.
systemd-run --scope --user failed to run in system 249.6, cf. #21297. Add tests
for systemd-run --scope and systemd-run --scope --user to make sure this does
not regress again.
