Prefix test target names with "ArgumentParser"

[elliottwilliams]

Similar to d8fb6bd and #18.

Prior to Swift 5.2, test target names need to be unique throughout the entire package graph. This change adds "SAP" "ArgumentParser" to each test target to avoid ambiguities.

Context: I'm trying to use swift-argument-parser in a package repo which also depends on Carthage, and they have a nested dependency with a UnitTests target. Since the argument parser is under active development and I assume I'm not the only one in this situation, I figured I would propose the change here.

Checklist

• [n/a] I've added at least one test that validates that my change is working, if appropriate
• [n/a] I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• [n/a] I've updated the documentation if necessary

[natecook1000]

Thanks for getting this workaround ready, @elliottwilliams! Looking at all the names, we'd rather go with an ArgumentParser prefix for everything. Would you mind adopting that prefix, including renaming SAPTestHelpers?

[elliottwilliams]

@natecook1000 sure thing, updated!

[natecook1000]

Thanks, @elliottwilliams! Merged locally.