[coll] Improve event loop. (#10199)

- Add a test for blocking calls.
- Do not require the queue to be empty after waking up; this frees up the thread to answer blocking calls.
- Handle EOF in read.
- Improve the error message in the result. Allow concatenation of multiple results.

doc/contrib/unit_tests.rst

@@ -144,6 +144,14 @@ which provides higher flexibility. For example:
 
   ctest --verbose
 
+If you need to debug errors on Windows using the debugger from VS, you can append the gtest flags in `test_main.cc`:
+
+.. code-block::
+
+  ::testing::GTEST_FLAG(filter) = "Suite.Test";
+  ::testing::GTEST_FLAG(repeat) = 10;
+
+
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 Sanitizers: Detect memory errors and data races
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