Fix: Avoid concurrent dialect patching in model testing

[VaggelisD]

To freeze time through execution_time in unit tests we essentially patch the SQLGlot dialects globally to generate any CURRENT_<date/time>() reference as a cast at the specified time & type.

This works fine if tests are run sequentially but runs into race conditions in concurrent execution. This PR:

• Replaces the global patching for SQL models by instead transforming the corresponding AST nodes before generation

• Locks the global patching for Python models using the runner's lock; This is required because Python models can execute any SQL statement, so the patching is indeed necessary to cover that globally.