Add tests to verify hairline config works properly after changes

Example/Tests/StackableTests.swift

@@ -0,0 +1,48 @@
+//
+//  StackableTests.swift
+//  
+//
+//  Created by Jonathan Cole on 12/1/21.
+//
+
+@testable import Stackable
+import XCTest
+
+class StackableTests: XCTestCase {
+
+    func testHairlineConfigLogic() {
+        // Test Global config
+        UIStackView.stackable.hairlineColor = .blue
+        let stack1 = UIStackView()
+        stack1.stackable.add([
+            UIStackView.stackable.hairline,
+        ])
+        let hairline1 =  stack1.arrangedSubviews.first as! StackableHairlineView
+        XCTAssert(hairline1.backgroundColor == .blue)
+
+        // Test instance config
+        let stack2 = UIStackView()
+        stack2.stackable.hairlineColor = .brown
+        stack2.stackable.add([
+            UIStackView.stackable.hairline,
+        ])
+        let hairline2 =  stack2.arrangedSubviews.first as! StackableHairlineView
+        XCTAssert(hairline2.backgroundColor == .brown)
+
+        // Test per-hairline Config
+        let stack3 = UIStackView()
+        stack3.stackable.hairlineColor = .brown
+        stack3.stackable.add([
+            UIStackView.stackable.hairline,
+            UIStackView.stackable.hairline
+                .color(.yellow),
+        ])
+        let hairline3 = stack3.arrangedSubviews.first as! StackableHairlineView
+        XCTAssert(hairline3.backgroundColor == .brown)
+
+        let hairline4 = stack3.arrangedSubviews.last as! StackableHairlineView
+        XCTAssert(hairline4.backgroundColor == .yellow)
+
+    }
+
+}