Fix source header license in tests

[mzweilin]

What does this PR do?

echo "# SPDX-License-Identifier: BSD-3-Clause" > new
find "tests" -type f -name "*.py" | xargs perl -i -p0e 's/# Licensed subject.*?and you.\n/`cat new`/se'
rm new

Fixes #97

Type of change

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• Improvement (non-breaking)
• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

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• Test A
• Test B

Before submitting

• The title is self-explanatory and the description concisely explains the PR
• My PR does only one thing, instead of bundling different changes together
• I list all the breaking changes introduced by this pull request
• I have commented my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have run pre-commit hooks with pre-commit run -a command without errors

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[dxoigmn]

LGTM!