Add severities attribute to per_file

[furtib]

Describe the task

We should have parity between the monolithic and per_file rules.
The per_file version of the rule currently does not accept severities as attributes and fails even for low-priority issues.

Scope

• Refactor per_file into a build and a test rule. (CodeChecker should only be called in the build rule, just as in the monolithic rule)
• Add a severities attribute to the pre_file rule. (Handle it the same way as monolithic, without CodeChecker)

Definition of Done

• Users can define severities for the per_file test target too.