[SLE15-SP6] Fixed comparing versions in the self-update version check

[lslezak]

Problem

• https://bugzilla.suse.com/show_bug.cgi?id=1234661
• YaST displays a false alarm error, there is actually no problem with the self-update repository
• The problem was in reading the versions from the /.packages.root file, the package version additionally contains the architecture at the end
• When comparing versions the "4.6.5-150600.3.6.1" version from the self-update looks older as "4.6.5-150600.3.6.1.x86_64" from the inst-sys so then YaST complains about downgrade
• This happens only when the installation is booted from PXE using the updated tftpboot-installation-* packages (not the GA version, that works fine)

Solution

• Remove the architecture suffix from the version

Testing

• Updated unit test
• Tested manually by the SUMA team

[coveralls]

coverage: 40.927% (+0.005%) from 40.922%
when pulling c1246bc on self_update_fix_SP6
into 0851b52 on SLE-15-SP6.

[yast-bot]

✔️ Internal Jenkins job #10 successfully finished
✔️ Created IBS submit request #356183