tests fail for Swift 5.1.3 release

[marc-medley]

swift --version
# Apple Swift version 5.1.3 (swiftlang-1100.0.282.1 clang-1100.0.33.15)
# Target: x86_64-apple-darwin19.3.0

swift test
# …/Vaux/Tests/VauxTests/VauxTableTests.swift:94:190: 
#   error: 'String' is not convertible to 'String?'
#       link(…, label: "…").class("external").attr("rel", "noopener")
#                                                         ^~~~~~~~~~
# …/Vaux/Tests/VauxTests/VauxTests.swift:1703:13: 
#   error: expression type 'HTML' is ambiguous without more context
#             fieldset {
#             ^~~~~~~~~~
# …/Vaux/Tests/VauxTests/VauxTests.swift:1714:28: 
#   error: 'InputType' is not convertible to 'InputType?'
#               input(type: .text)
#                           ~^~~~
# …/Vaux/Tests/VauxTests/VauxTests.swift:2386:49: 
#   error: 'String' is not convertible to 'String?'
#               }.attr("for", "a b").attr("name", "x")
#                                                 ^~~
# …/Vaux/Tests/VauxTests/VauxTests.swift:2386:29: 
#   error: 'String' is not convertible to 'String?'
#               }.attr("for", "a b").attr("name", "x")
#                             ^~~~~