chore: add spdx license identifier header to test modules

[afuetterer]

This PR adds a spdx license identifier header to the test modules I created prior. This adds the license information, without being as verbose as the other modules.

What do you think?

Ref: https://spdx.dev/learn/handling-license-info/

[coveralls]

coverage: 67.553%. remained the same — afuetterer:license into chrismattmann:master

[afuetterer]

Would you like to replace the verbose license header in the other modules with this one-liner as well?

I am unsure if you really need this boilerplate. There is no NOTICE file, that is mentioned in the header.

# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License.  You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

Is this library really licensed to the ASF or does it just use the Apache license?