repl_writer_thread_count_param.js sometimes fail because of an assert when grepping logs for a certain message, see BF-40992.

Assert failure is here: https://github.com/10gen/mongo/blob/40c8f7d482e0d2b63fedcfaaaf4858958604f4f7/jstests/noPassthrough/replication/repl_writer_thread_count_param.js#L356

We expect the thread pool to spawn new threads when increasing the replWriterMinThreadCount param, but from the log, the new threads are spawned right after the assert fails (see logs). My guess is that sometimes the logs get written to the file/output a bit slower than usual, resulting in the assert being executed before the logs get actually written.

The solution is to use assert.soon() so as to give time for the logs to be flushed.