testSubprocessPlatfomOptionsPreSpawnProcessConfigurator fails on Linux

[jakepetroules]

Describe the bug
testSubprocessPlatfomOptionsPreSpawnProcessConfigurator may fail because it assumes it can set a group ID of 4321, which might not exist on the running system.

It should try an alternative technique to find a group ID that exists on the system and use that (use getent group and pick one?).

Additionally, we should #expect that setgid returns 0.

To Reproduce
swift test --filter testSubprocessPlatfomOptionsPreSpawnProcessConfigurator

Expected behavior
Test should pass

Environment (please complete the following information):

• Linux debian 6.1.0-31-arm64 #\1 SMP Debian 6.1.128-1 (2025-02-07) aarch64 GNU/Linux
• Debian GNU/Linux 12.9 (bookworm)
• Swift version 6.1 (swift-6.1-RELEASE)
• Target: aarch64-unknown-linux-gnu

Additional context
Add any other context about the problem here.

[iCharlesHu]

This seems to be a Debian specific issue. This test passes fine for me on Ubuntu 22.04

[jakepetroules]

Yes and no -- it might work on Ubuntu today, but relying on a specific group ID to exist is fundamentally unstable.

[jakepetroules]

Oddly enough, sometimes this seems to pass, even in cases where group 4321 doesn't exist.