Set availability for inverted character class test

This feature depends on running with a Swift 5.7 stdlib, and fails
when that isn't available.

Author: natecook1000

Tests/DocumentationTests/RegexBuilderTests.swift

@@ -21,6 +21,8 @@ class RegexBuilderTests: XCTestCase {}
 
 extension RegexBuilderTests {
   func testCharacterClass_inverted() throws {
+    guard #available(macOS 13, *) else { return }
+    
     let validCharacters = CharacterClass("a"..."z", .anyOf("-_"))
     let invalidCharacters = validCharacters.inverted