Replace deprecated spin_some in realtime_tools (backport #448)

[mergify]

Overview

This PR replaces uses of the deprecated rclcpp::spin_some(node) with rclcpp::executors::SingleThreadedExecutor in the realtime tools tests, addressing issue #411

What changed

Files changed are:

• realtime_tools/test/realtime_server_goal_handle_tests.cpp
• realtime_tools/test/realtime_publisher_tests.cpp

Notes for Reviewer

Happy to make any changes that are requested. Please also double check my approach with the spin_some() usage in the TEST(RealtimeServerGoalHandle, set_canceled) test.

---

This is an automatic backport of pull request #448 done by Mergify.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.44%. Comparing base (47d4779) to head (91b82ab).
⚠️ Report is 1 commits behind head on kilted.

Files with missing lines	Patch %	Lines
...e_tools/test/realtime_server_goal_handle_tests.cpp	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           kilted     #451      +/-   ##
==========================================
+ Coverage   85.34%   85.44%   +0.10%     
==========================================
  Files          17       17              
  Lines        1440     1450      +10     
  Branches      135      135              
==========================================
+ Hits         1229     1239      +10     
  Misses        122      122              
  Partials       89       89              

Flag	Coverage Δ
unittests	85.44% <87.50%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
realtime_tools/test/realtime_publisher_tests.cpp	88.88% <100.00%> (+0.31%)	⬆️
...e_tools/test/realtime_server_goal_handle_tests.cpp	92.80% <84.61%> (+0.43%)	⬆️

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