ipatests: replace ad hoc backup with FileBackup helper

[wladich]

Test test_smb_mount_and_access_by_different_users was failing with message
```
kdestroy: Permission denied while initializing krb5
```

This happened because the previous test
`test_smb_access_for_ad_user_at_ipa_client` was calling the fixture
`enable_smb_client_dns_lookup_kdc` which was doing backup of krb5.conf
in a wrong way:
- mktemp (to create a temp file)
- cp /etc/krb5.conf to the temp file
- ...
- mv tempfile /etc/krb5.conf

This flow looses the file permissions, because mktemp creates a file
using the default umask, which results in -rw------- permissions.
The copy does not modify the permissions, and the mv keeps the
permissions from the source => /etc/krb5.conf now has -rw-------.

Fixes: https://pagure.io/freeipa/issue/8115

[frozencemetery]

Nit: it's "ad hoc", not "ad-hock". "ad hoc" is from Latin; it means roughly "for this purpose", or in English, is equivalent to "bespoke" or "custom-built". It's unrelated to the word "hock", which has a variety of meanings but is most commonly used around debt.

[flo-renaud]

@wladich
Thanks for the fix, LGTM. If you could fix the typo noted by @frozencemetery in the commit msg that would be great.

[wladich]

I was sure I fixed it yesterday, but something went wrong. Fixed now.

[wladich]

Fix for FileBackup is backported to ipa-4-7 in #3883

[abbra]

LGTM.

[wladich]

master:

• 72540c4 ipatests: refactor FileBackup helper
• c2b230c ipatests: replace ad hoc backup with FileBackup helper
• f58fb57 ipatests: enable test_smb.py in gating.yaml