tests: Increase unit-test coverage on create.go
tests: Increase unit-test coverage on create.go

Add a new test `TestCreateCgroupsFilesFailToWriteFile()` to test another
failure path.

Fixes #485.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

clearcontainers/runtime,devimc/runtime,devimc/runtime,chavafg/runtime,clearcontainers/runtime,chavafg/runtime