tests: Fix fork/exec test error

Fixed `TestSandboxCreationFromConfigRollbackFromCreateSandbox` which
requires that the hypervisor does not exist. Unfortunately, it does
exist (as a fake test binary), but isn't executable meaning although the
test failed (since an error is expected), rather than the expected
`ENOENT` error, the test was logging a message similar to the following
since the fake hypervisor exists with non-executable permissions:

```
Unable to launch /tmp/vc-tmp-526112270/hypervisor: fork/exec /tmp/vc-tmp-526112270/hypervisor: permission denied
```

Fixes: kata-containers#1835.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

virtcontainers/sandbox_test.go

@@ -1530,6 +1530,10 @@ func TestSandboxCreationFromConfigRollbackFromCreateSandbox(t *testing.T) {
 		Volumes:          nil,
 		Containers:       nil,
 	}
+
+	// Ensure hypervisor doesn't exist
+	assert.NoError(os.Remove(hConf.HypervisorPath))
+
 	_, err := createSandboxFromConfig(ctx, sConf, nil)
 	// Fail at createSandbox: QEMU path does not exist, it is expected. Then rollback is called
 	assert.Error(err)