Mention test-concurrent-safe CI job in CONTRIBUTING doc (#5781)

Related #5759

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Co-authored-by: Robert Pająk <pellared@hotmail.com>

CONTRIBUTING.md

@@ -578,7 +578,10 @@ See also:
 The tests should never leak goroutines.
 
 Use the term `ConcurrentSafe` in the test name when it aims to verify the
-absence of race conditions.
+absence of race conditions. The top-level tests with this term will be run
+many times in the `test-concurrent-safe` CI job to increase the chance of
+catching concurrency issues. This does not apply to subtests when this term
+is not in their root name.
 
 ### Internal packages