plugins: fixed tests for dependency graph

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: added stub class PluginDependencyGraphCycleFinder

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: made PluginDependencyGraphCycleFinder find simple cycles

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: added more tests for cycle detection

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: added selfDependencyTest

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>