Fix lookup for certicates in tests

The file lookup is by default setting `rstrip=True` which could lead
into a stripped new line. This is not happening always but resulted in
failed tests sometimes with certificates pasted to the b64encode filter.

For calls of lookup in the certificae tests `rstrip=False` has been
added to make sure that this is not happening any more. Not in
test_dnsrecord as lookup(..., rstrip=False) is adding a new line if
there was not a new line and this is an issue for dnsrecord. The user
and host tests have also been simplified to create the base64 encoded
file in the beginning and use this file then later on in the tests
without the need to use the b64encode filter.

Ref: https://github.com/ansible/ansible/issues/57521#issuecomment-502238000