tests: do cleanUp() always in the end

Fixes: kata-containers#1422

Detect failing test case:

```
....
=== RUN   TestEnterContainerFailingContNotStarted
--- PASS: TestEnterContainerFailingContNotStarted (0.01s)
=== RUN   TestEnterContainer
--- FAIL: TestEnterContainer (0.00s)
 Error Trace: sandbox_test.go:1154
 Error:      	Expected value not to be nil.
 Messages:   	Entering non-running container should fail
 Error Trace: sandbox_test.go:1157
 Error:      	Expected nil, but got: &errors.errorString{s:"Can not
move from running to running"}
 Messages:   	Failed to start sandbox: Can not move from running to
running
FAIL
```

`TestEnterContainerFailingContNotStarted` calls `cleanUp` at function
begging but it doesn't clean its garbage after it ends.
`TestEnterContainer` only call `cleanUp` in the end but it doesn't do
cleanUp in the begging, that gives first test case a chance to impact
latter one.

This commit modifies all the test cases, let them all do the cleanUp()
in the end.

The policy here is: "everyone needs to take their garbage away when they
leave" :)

Signed-off-by: Wei Zhang <[email protected]>