docs(test-utils): fix package name in readme

[JamieDanielson]

Which problem is this PR solving?

• confusion about what package to install from npm

Short description of the changes

• Looks like this was renamed from test-utils to contrib-test-utils a few years back but the readme still uses the old name. I happened across it when I was looking for the test code and was confused about the package name. This is super tiny, merge at will.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.42%. Comparing base (dfb2dff) to head (72a8360).
Report is 189 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2338      +/-   ##
==========================================
- Coverage   90.97%   90.42%   -0.56%     
==========================================
  Files         146      148       +2     
  Lines        7492     7333     -159     
  Branches     1502     1524      +22     
==========================================
- Hits         6816     6631     -185     
- Misses        676      702      +26     

see 62 files with indirect coverage changes