[libc++] Relax libc++-only test on regex_constants

The standard requires the following for the std::regex_constants::error_type
values: "The type error_type is an implementation-defined enumerated type."

The values of this enumerated type are not required to be non-zero.
This patch makes such checks in tests libc++-specific to let the tests
pass for other conforming implementations.

Thanks to Andrey Maksimov for the patch.
Differential Revision: https://reviews.llvm.org/D61195

llvm-svn: 359320