Test discovery on Linux

[0xTim (Tim)]

@dlbuckley

Unfortunately I've found that just running swift build still requires the LinuxMain.swift file to exist, even though it's not needed any more. I'm not sure why just doing a plain build without the tests requires the LinuxMain.swift file. Is there a reason for this and are there any plans to change it in future?

Just hit this issue when building an app inside Docker that's had the LinuxMain and all the allTests stripped out as it's no longer needed. A completely empty LinuxMain.swift fixes the issue but as mentioned it seems weird to require it for the build to pass (I'm assuming it's because the build command also attempts to build the Test target as well). It also isn't required when building on macOS. Another fix is to build it with the --enable-test-discovery flag as well.