Add PrebuildClean command unit test - #2770

Summary

Add unit coverage for the Expo Prebuild Clean command.

Proposed Changes

• Add a PrebuildClean command unit test mirroring the existing Prebuild command coverage.
• Verify the command uses the Expo Prebuild Clean logger channel.
• Verify the node modules root lookup and command executor invocation use the project root.
• Verify the exact command string is
  px expo prebuild --clean.

Test Plan

• pm run build
• pm test (fails in existing n-extension commandsRegistered test because the registered command list includes
  eactNative.publishToExpHost; the new prebuildCleanCommand test passes.)

Closes #2755