Deadlock when `assertSnapshot` called on main thread but not on main queue

[jcavar]

Describe the bug

This commit changes the approach to detect if a snapshot is generated on the main thread. However, as discussed in GRDBSnapshotTests issue, this is a breaking change for certain code patterns.

To Reproduce

Download GRDBSnapshotTesting and add the following test case:

// Crash
func test_issue_5() throws {
    let dbQueue = try DatabaseQueue()
    try dbQueue.read { db in
        assertSnapshot(of: "SELECT 'Hello, world!'", as: .dump(db))
    }
}

Or:

// On the main thread
let queue = DispatchQueue(label: "test")
queue.sync {
    print("Hello")

    // Success: we're on the main thread
    assert(Thread.isMainThread)

    // assertSnapshot ...
}

Expected behavior
A snapshot is successfully recorded and generated

Environment

• swift-snapshot-testing version 1.18.0 and higher
• Xcode 16.2
• Swift 6
• OS: iOS 18