ReaderTestSupport duplicated between builtins and reader test modules

[gnodet]

ReaderTestSupport exists as a test class in both:

• reader/src/test/java/org/jline/reader/impl/ReaderTestSupport.java
• builtins/src/test/java/org/jline/builtins/ReaderTestSupport.java

The builtins copy is a near-duplicate maintained separately. This creates a maintenance burden — changes to one must be manually replicated to the other.

Proposed action:

• Extract ReaderTestSupport into a shared test-utilities module or use a test-jar dependency
• Remove the duplicate from builtins

Found during review of #1794.

Claude Code on behalf of Guillaume Nodet