test: Add test utils in studio-components

[TomasEng]

Description

Added a directory for test utils in the component library. It contains functions that are common in several component tests.

• testCustomAttributes to test forwarding of "rest" props
• testRefForwarding to test forwarding of ref objects and functions
• testRootClassNameAppending to test that the given class is appended to the default class list of the root element

Related Issue(s)

• Implement new StudioCodelistEditor component #13342

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (b96cae8) to head (0943cb2).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13535   +/-   ##
=======================================
  Coverage   95.15%   95.16%           
=======================================
  Files        1458     1462    +4     
  Lines       20123    20162   +39     
  Branches     2438     2440    +2     
=======================================
+ Hits        19149    19188   +39     
  Misses        868      868           
  Partials      106      106           

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[framitdavid]

Rock Solid! 🪨 This would make it a lot easier to test repeatable stuff! 👏