gh-152548: Convert test_eintr to the @isolated() decorator

The EINTR tests need a process with no background thread so signals reach
the main thread; they were driven as a script via run_test_script.  With
@isolation.isolated() each test class runs in its own fresh subprocess and
the tests are reported individually instead of as one opaque test_all.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka