docs: cleanup aftert test should remove namespace, Closes #6791

[m-szalik]

Two examples have the same issue.
Should be: namespace should be removed at the end of the test
It is: namespace is created again

• operator-sdk/testdata/go/v4/memcached-operator/test/e2e/e2e_test.go

  Line 87 in e95abdb

  cmd := exec.Command("kubectl", "create", "ns", namespace)

• operator-sdk/testdata/go/v4/monitoring/memcached-operator/test/e2e/e2e_test.go

  Line 103 in e95abdb

  cmd := exec.Command("kubectl", "create", "ns", namespace)