Update tests for latest mini_mime update

[lildude]

Description

I noticed merging #5287 today resulted in two unrelated test failures that didn't occur before yesterday:

  1) Failure:
TestBlob#test_mime_type [/home/runner/work/linguist/linguist/test/test_blob.rb:17]:
Expected: "application/postscript"
  Actual: "application/pdf"

  2) Failure:
TestFileBlob#test_mime_type [/home/runner/work/linguist/linguist/test/test_file_blob.rb:31]:
Expected: "application/postscript"
  Actual: "application/pdf"

MiniMime released a new version yesterday which has pulled in the latest updates from https://github.com/mime-types/mime-types-data which has in turn pulled in @Alhadis's fix to correctly classify .ai files as PDFs in mime-types/mime-types-data#22

This PR updates our tests to reflect these changes.

[ Checklist removed as it doesn't apply ]

[Alhadis]

I was wondering about this quite recently…