[test] Drop stdlib-redundant tests
[test] Drop stdlib-redundant tests

hasPrefix/hasSuffix is now available universally from the standard
library (it used to be Darwin only). As such, these tests are
redundant with the tests in the standard library (indeed they are
literally copy-pasted from the stdlib). As test methodology changes,
maintaining these across projects and platforms is troublesome.

alblue/swift-corelibs-foundation,alblue/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,alblue/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,alblue/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,apple/swift-corelibs-foundation,alblue/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation