Fix hanging and failing tests in SwiftBarTests

[melonamin]

Summary

The SwiftBarTests scheme hosts SwiftBarTests.xctest inside SwiftBar.app, so AppDelegate.applicationDidFinishLaunching runs in the test process. Three things kept it running long after the last @Test completed and prevented the host from terminating, which made xcodebuild test hang indefinitely after every test actually finished in milliseconds:

• Sparkle's SPUUpdater registers persistent runloop sources for update checks.
• The missing-plugin-folder branch shows a modal NSAlert that the test runner cannot dismiss.
• PluginManager.loadPlugins() spawns plugin processes, file-system observers, and timers.

Detect the XCTest environment via XCTestConfigurationFilePath and skip the heavy setup, keeping only the lightweight pluginManager assignment that one integration test references via delegate.pluginManager.

Also fix testSyncFilePlugins_keepsSymlinkedPackagedPluginMatchedByBundlePath, a pre-existing failure: FileManager.contentsOfDirectory(at:) does not follow a directory-typed URL when the URL itself is a symlink (returns ENOTDIR). Resolve the symlink inside PackagedPlugin.findMainExecutable before enumerating, while keeping the .swiftbar extension check on the unresolved URL. In packagedPluginFileState pass the unresolved packageURL to findMainExecutable for the same reason.

Before/after

• Before: xcodebuild test runs the test cases in <1s but the test process stays alive indefinitely (observed >30 min) until killed.
• After: full suite runs in ~4s, 164/164 passing.

Test plan

• xcodebuild test completes cleanly with ** TEST SUCCEEDED **
• Previously-failing testSyncFilePlugins_keepsSymlinkedPackagedPluginMatchedByBundlePath now passes
• Production launch path unaffected (the test-mode guard returns early only when XCTestConfigurationFilePath is set)