[RFE] Add documentation on running the test suite locally

[t-lo]

Current situation

Running the qemu test suite locally is pretty straightforward for knowledgeable maintainers. Since it is undocumented it's nigh impossible for the uninitiated to leverage Flatcar's ci automation to run test though.

Impact

Users and contributors don't know that running tests locally is actually very easy.

Ideal future situation

Running tests is documented in https://github.com/flatcar/flatcar-docs .

Implementation options

• cover ci automation testing
• cover manual kola runs (advanced testing)

Additional information

Some hints on running tests locally using the ci automation scripts can be taken from https://github.com/flatcar/scripts/blob/main/.github/workflows/run-kola-tests.yaml#L198.

Running individual tests with plain kola is documented here: https://github.com/flatcar/mantle/tree/flatcar-master#kola-run .

Some tests (devcontainer, torcx, etc) will fail, but the tl;dr is

source ci-automation/ci_automation_common.sh
source sdk_container/.repo/manifests/version.txt
# adjust to higher or lower value depending on system RAM
export PARALLEL_TESTS=4
export QEMU_KOLA_SKIP_MANGLE=true

test_run amd64 qemu_uefi