Describe the bug
When running tests of version 2.1.4 in Debian we get the following failure:

  1) Vips::SourceCustom can write an image to a user output stream     Failure/Error: expect(image.avg).to be_within(0.001).of(109.789)       expected 109.41407425505706 to be within 0.001 of 109.789

This happens all the time, which makes me think if the range (0.001) is indeed correct.

To Reproduce
Steps to reproduce the behavior:

1. Grab the debian package for ruby-vips version 2.1.4 (latest version)
2. Try to build it.

I believe if you simple try to run tests in Debian unstable you will get the same failure. Currently the default is ruby 3.1.

Expected behavior
All tests pass.

Desktop (please complete the following information):

• OS: Debian
• Version: unstable (sid)