Bug 21333 – runModuleUnitTests inner code should be available as a function

Currently, in order to affect the behavior of the default unit tester, you must reimplement it. In this code: https://github.com/dlang/druntime/blob/86e5cf3fa15afda116994da0c91f40aa7a5e6c6e/src/core/runtime.d#L562 If you define a default unit tester, it skips the normal running of unittests. The meat of actually running the unit tests should be abstracted into a function, such that you can call it but just affect the result (like if you don't want to run main, but you want to suppress a successful output). Simple fix, just abstract the unit test running into its own function.

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