The integration test TestExportContainerWithOutputAndImportImage in moby/moby is the same as TestExportContainerAndImportImage,
except for the output file option. Adding a unit test to cover the output file option of the export command here allows
the removal of the redundant integration test TestExportContainerWithOutputAndImportImage.
Signed-off-by: Arash Deshmeh <adeshmeh@ca.ibm.com>