tests: Add unit tests for exec.go
tests: Add unit tests for exec.go

Use mock framework to implement tests for exec.go

Fixes #398

Signed-off-by: Archana Shinde <047ac45ce396f63f3cf93ff43725668e736e0632@intel.com>

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