Tests: adjust test for Windows (#29)

The Windows environment has a smaller stack limit.  Without
optimizations, we can manage to run up to ~950 recursive calls.  With
release mode, we are able to get to ~1250.  Limit the artificial call
depth accordingly.  This allows the test suite to pass on Windows.

Author: compnerd

Tests/MarkdownTests/Performance/EditPerformanceTests.swift

@@ -12,7 +12,15 @@ import XCTest
 import Markdown
 
 final class EditPerformanceTests: XCTestCase {
+#if os(Windows)
+#if DEBUG
+    static let maxDepth = 625
+#else
+    static let maxDepth = 1250
+#endif
+#else
     static let maxDepth = 5000
+#endif
     /// Test the performance of changing a leaf in an unrealistically deep markup tree.
     func testChangeTextInDeepTree() {
         func buildDeepListItem(depth: Int) -> ListItem {