We introduce a new test scenarios header `check` that allows
to mark test scenarios as specific to a single check engine
type. For example, adding header `check = sce` to a test scenario
marks this test scenario as specific only to SCE. If SCE check
isn't available, such scenario will be skipped. If the `check`
header isn't specified in a test scenario, the test scenario
will work with any check type.

Fixes: ComplianceAsCode#12030

For SCE test scenarios we will check if SCE is supported by
OpenSCAP installed in the testing back end. If OpenSCAP on the
back end doesn't support SCE we will display a warning and skip
the tests.

SCE support will be verified only once, during the back end
initialization.