fix: Create test module to test SPI

[kewne]

Adds a "test" module for testing the SPI.

The reason for this is that, due to the changes in how ServiceLoader works, META-INF/services files in test source trees are put in unnamed modules and thus cannot be accessed by the ServiceLoader in the org.javamoney.moneta module.

This should fix the issues from #370.

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This change is 

[keilw]

Thanks, @kewne, there are many files changed by this PR. We appreciate your contribution, would you consider joining the JCP as an associate member if you plan to contribute on a similar scale more often? As far as I can see, you are not a JCP member yet, joining is free on every level.

[kewne]

@keilw Most of the changes are copying files into the new module. In many cases we can probably remove the originals. Let me know if you think I should make any changes here or create a follow-up PR.

I'm definitely interested in contributing to this and other projects as I can, I'll look into it. Thanks!

[keilw]

@kewne That's fine, if you can, please mark the original files @deprecated either via annotation or JavaDoc.

[kewne]

@keilw Done, I've additionally moved TestRoundingProvider into src/main/java, so it's closer to how it's really used.

[kewne]

@keilw I've renamed the module as requested.