Adding method to reset the mocked phpmailer instance

[welcher]

Added a new decorator method to rest the mock phpmailer object to make it more clear as to what the code is doing. Also, I removed the tests_ prefix from the retrieve_phpmailer_instance method as it is not needed and may cause confusion as it does not contain any tests.

[boonebgorges]

The reset method looks good to me.

I think the tests_ prefix on the function is meant to be a namespace - see tests_add_filter(). I am wary of changing it just for aesthetic reasons, as there may be third-party libraries using it. Could you change it back and post an updated patch?

[welcher]

Re-adding tests_ prefix

[welcher]

@boonebgorges I've added the tests_ prefix back in.

[boonebgorges]

In 37358:

Tests: Introduce reset_phpmailer_instance() function.

This function provides a more convenient method for resetting the
PHPMailer instance than the previous technique of reaching into the global.

Props welcher.
Fixes #36658.