Reduce memory required by test_masked_registration_3d_contiguous_mask

[grlee77]

Description

Recently, I saw this test failure that looks like an out of memory error occurred:
https://dev.azure.com/scikit-image/scikit-image/_build/results?buildId=7008&view=logs&jobId=c266ee3e-f298-5c55-b78e-d135f25941c2&j=c266ee3e-f298-5c55-b78e-d135f25941c2&t=76d8336e-2ec7-54e7-5531-ad7a9096482b

I think it came up once previously as well. In this PR, I just reduce the data size by a factor of 4 in that test case to reduce the memory required.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[grlee77]

failure to allocate error was also observed on Azure in #6034: https://dev.azure.com/scikit-image/scikit-image/_build/results?buildId=7072&view=logs&j=0c528f75-1409-556f-1999-9f155ac37a3a&t=1c50252d-14f6-55f6-616c-dfe500c925e4&l=7511

[grlee77]

@scikit-image/core. Is anyone available to review this small PR? I think it may fix the Python 3.7 failures on Azure seen in multiple recent PRs.

[alexdesiqueira]

LGTM. Thank you @grlee77!