[libcxx][test][NFC] Various tests for std::vector

Add missing tests for std::vector funcionality to improve code coverage:

  - Rewrote access tests to check modification of the container using
    the reference returned by the non-const overload
  - Added tests for reverse iterators: rbegin, rend, etc.
  - Added exception test for vector::reserve
  - Extended test cases for vector copy assignment
  - Fixed insert_iter_value.pass.cpp to use insert overload with const
    value_type& (not with value_type&& which is tested in
    iter_rvalue.pass.cpp test)

Reviewed By: Quuxplusone, rarutyun, #libc

Differential Revision: https://reviews.llvm.org/D112438