Investigate "Known Issues" feature of SQUAD (mark tests as "allowed to fail")
## Issue Background
SQUAD offers the possibility to mark tests as `has_known_issues`. A test also features the field `issues`.
We would like to know if this can be used to marks tests, e.g. to be able to identify them as unimportant or known to fail.
## Issue description
Find out how to add a known issues and what it actually means for the SQUAD reporting.
This should be documented somewhere, possibly together with a script or tool, that shows how to mark known issues.
## Acceptance criteria
- [x] It is documented how to create a known issue
- [ ] A small script is provided, that shows how to create a new known issue
- [x] The effect on the test reports of known issues has been clarified and documented
- [x] It is clarified if the known issues feature in SQUAD can be used to mark tests as *allowed to fail*
## Outcome
- Known issues can be created using the administration panel:
- Required fields:
- test suite and test name: `<suite-name>/<test-name>`, e.g. `spectre-meltdown-checker-test/CVE-2018-3640`)
- One or more group/project/environment combinations: `<group-name>/<project-name> - <environment-name>`, e.g. `linux-cip/linux-6.1.y-cip - arm_renesas_shmobile_defconfig_r8a7743-iwg20d-q7`
- The known issues tab gives an overview of tests with known issues (see e.g. [here](https://squad.ciplatform.org/cip-kernel/linux-6.1.y-cip/knownissues/))
- Known issues are not registered at previously reported tests. E.g. [this test](https://squad.ciplatform.org/api/tests/169542/) has known issues, but is not registered.
- Test result reporting, after a known issue has been created, are marked as `xfail`. See for example [this result](https://squad.ciplatform.org/cip-kernel/linux-6.1.y-cip/build/6.1.62-cip9_4441e8879/testrun/11508/suite/spectre-meltdown-checker-test/test/CVE-2018-3640/details/)
issue