selftests: tc-testing: add "depends_on" property to skip tests

currently, users can skip individual test cases by means of writing

  "skip": "yes"

in the scenario file. Extend this functionality, introducing 'dependsOn':
it's optional property like "skip", but the value contains a command (for
example, a probe on iproute2 to check if it supports a specific feature).
If such property is present, tdc executes that command and skips the test
when the return value is non-zero.

Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>