Add integration tests for inofity limits

inotify exposes a /proc interface to limit kernel memory usage. If those
limits are set too low, inotify cannot add all watches. The integration
test verifies, that Inotify yields an error in that case.

Tests/InotifyIntegrationTests/Utilities/createSubdirectorytree.swift

@@ -0,0 +1,53 @@
+import Foundation
+import SystemPackage
+
+func createSubdirectorytree(at dir: String, foldersPerLevel: Int, levels: Int) throws {
+	let fileManager = FileManager.default
+
+	for path in SubfolderTreeIterator(basePath: dir, foldersPerLevel: foldersPerLevel, levels: levels) {
+		try fileManager.createDirectory(
+			at: path,
+			withIntermediateDirectories: true,
+			attributes: nil
+		)
+	}
+}
+
+struct SubfolderTreeIterator: IteratorProtocol, Sequence {
+	let basePath: URL
+	let foldersPerLevel: Int
+	let levels: Int
+	private var indices: [Int]
+	private var done = false
+
+	init(basePath: String, foldersPerLevel: Int, levels: Int) {
+		self.basePath = URL(filePath: basePath)
+		self.foldersPerLevel = foldersPerLevel
+		self.levels = levels
+		self.indices = Array(repeating: 1, count: levels)
+	}
+
+	mutating func next() -> URL? {
+		guard !done else { return nil }
+
+		let path = indices.reduce(basePath) { partialPath, index in
+			partialPath.appending(path: "Folder \(index)")
+		}
+
+		// Advance indices (odometer-style, rightmost increments first)
+		var carry = true
+		for i in (0..<levels).reversed() {
+			if carry {
+				indices[i] += 1
+				if indices[i] > foldersPerLevel {
+					indices[i] = 1
+				} else {
+					carry = false
+				}
+			}
+		}
+		if carry { done = true }
+
+		return path
+	}
+}