XCTest plugin should offer more convenient support for testing on Linux

Currently, a separate "main.swift" Swift source file is required for tests to run on Linux. This has two usability issues:

- Gradle expects "main.swift" whereas the Swift PM convention is "LinuxMain.swift". It would be easier to migrate if Gradle used the same convention or made this configurable.
- The "main.swift" source file should not be compiled on MacOS, and build logic rather than the XCTest plugin is currently responsible for dealing with this. It would be better to move this to the plugin.

Although a better solution would be to remove the need for a "main.swift" entirely, in the short term we can make some minor tweaks to improve the experience.

Start by examining the XCTest plugin and the build logic that currently handles main.swift on Linux and macOS. Compare the existing main.swift convention with Swift Package Manager's LinuxMain.swift convention. Done means the plugin provides more convenient Linux test support, including configurable or matching source naming and keeping the Linux entry-point source out of macOS compilation.

Written by the indexing model from the issue text.