[libcxx][test][NFC] Extend testing for vector.cons

Extending std::vector tests in vector.cons module:

- std::vector::assign when source range is bigger than destination
  capacity
- construction of empty vector using copy ctor, initializer_list ctor and
  others

Reviewed By: ldionne, rarutyun, #libc

Differential Revision: https://reviews.llvm.org/D114954