Always ensure that stderr is empty after executing scripts

[0xced]

What does this PR do?

This pull request add assertions to all tests that call ExecScriptAsync asserting that no error was written to stderr.

Adding these assertions found two issues, in the MySQL and MongoDB implementations.

MySQL would produce the following warning on stderr:

mysql: [Warning] Using a password on the command line interface can be insecure.

This has been addressed in 925f426081d8188dcc510898692f37db6e2933b2 by writing the configuration into /etc/mysql/my.cnf instead of passing it on the command line.

MongoDB would produce the following error on stderr (when using the mongo:4.4 image in the MongoDbV4Configuration test):

/bin/sh: 1: mongosh: not found

This has been addressed in 3ceeb4b02272d1f9d444fbc35e408e4fc2127192 by verifying if the mongosh binary exists and choosing the proper binary instead of running mongosh || mongo through /bin/sh. This also simplified the mongosh command implementation.

Finally, I also fixed some typos: // When should be // Then in the assertion part of the tests.

Why is it important?

Having stderr empty after successfully executing a script ensures that stderr will only contain actual errors and not spurious warnings or errors that are implementation detail errors that don't actually matter.

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[HofmeisterAn]

Thanks again.