Improve FunctionDefSpec

[jczuchnowski]

We need to make the tests between different modules more unified and consistent.

Starting with FunctionDefSpec (for every db-module):

• split the FunctionDefSpec into CommonFunctionDefSpec and CustomFunctionDefSpec
• CommonFunctionDefSpec should containt tests for all common functions defined in the Expr module https://github.com/zio/zio-sql/blob/master/core/jvm/src/main/scala/zio/sql/expr.scala#L389
• CustomFunctionsDefSpec should contain tests for the db-specific functions defined in each db-specific *SqlModule
• code duplication between the dbs is fine for CommonFunctionDefSpec unless there's another way to deal with it (base test class?)