[TestSupport] Unset the env variable that allows skipping tests
[TestSupport] Unset the env variable that allows skipping tests

Since this effects the tests that use swift-test.

apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager