Allow test checks for the shell discover step

[lukaszachy]

Checks have no effect, for example:

discover:
    how: shell
    tests:
    - test: echo
      name: /
      check:
      - how: avc

[psss]

I guess this should be relatively easy to implement. Mainly extending the TestDescription class:

tmt/tmt/steps/discover/shell.py

Lines 29 to 33 in d3497b5

	class TestDescription(
	SpecBasedContainer[dict[str, Any], dict[str, Any]],
	tmt.utils.NormalizeKeysMixin,
	SerializableContainer,
	):

With the check key definition:

tmt/tmt/base.py

Lines 1183 to 1189 in d3497b5

	check: list[Check] = field(
	default_factory=list,
	normalize=tmt.checks.normalize_test_checks,
	serialize=lambda checks: [check.to_spec() for check in checks],
	unserialize=lambda serialized: [Check.from_spec(**check) for check in serialized],
	exporter=lambda value: [check.to_minimal_spec() for check in value],
	)

Plus extending the schema.

[psss]

Prerequisite for the global avc check, blocking users which have tests defined in the discover step. Let's fix this as fast as possible. The proper solution to prevent the duplication to be added to the backlog.

[happz]

Prerequisite for the global avc check, blocking users which have tests defined in the discover step. Let's fix this as fast as possible. The proper solution to prevent the duplication to be added to the backlog.

#3621