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

@@ -20,6 +20,7 @@ import RegexBuilder
 class RegexBuilderTests: XCTestCase {}
 
 extension RegexBuilderTests {
+  @available(swift 5.7)
   func testCharacterClass_inverted() throws {
     let validCharacters = CharacterClass("a"..."z", .anyOf("-_"))
     let invalidCharacters = validCharacters.inverted