misc: fixed graph find-cycle alghoritm

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

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: simplify tests for PluginDependencyGraph

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: simplify tests for graph-sort-successors

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

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