Test failures and compiler warnings

[ellert]

• Fix some compiler warnings

  .../src/scitokens_internal.h:673:25: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
    673 |         return std::move(result);
        |                ~~~~~~~~~^~~~~~~~
  .../src/scitokens_internal.h:673:25: note: remove 'std::move' call
  
  .../src/scitokens_internal.cpp:633:25: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
    633 |         return std::move(status);
        |                ~~~~~~~~~^~~~~~~~
  .../src/scitokens_internal.cpp:633:25: note: remove 'std::move' call
  
  .../src/scitokens_internal.cpp:747:13: warning: unused variable 'next_update' [-Wunused-variable]
    747 |     int64_t next_update, expires;
        |             ^~~~~~~~~~~
  
  .../src/scitokens_internal.cpp:747:26: warning: unused variable 'expires' [-Wunused-variable]
    747 |     int64_t next_update, expires;
        |                          ^~~~~~~
  
  .../test/main.cpp:442:12: warning: unused variable 'now' [-Wunused-variable]
    442 |     time_t now = time(NULL);
        |            ^~~

• Move key cache tests to the end to avoid random failures in other tests due to interference from changing the cache expiration settings in the key cache tests.

• Print err_msg in case of error

• Use resize() instead of reserve() to allocate vector elements

  [ RUN      ] SerializeTest.VerifyTest
  /usr/include/c++/13/bits/stl_vector.h:1125: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = unsigned char; _Alloc = std::allocator<unsigned char>; reference = unsigned char&; size_type = long unsigned int]: Assertion '__n < this->size()' failed.

[olifre]

Many thanks, @ellert !

To weigh in on this, the std::vector issue has also popped up on Gentoo Hardened systems, so we'll now also have to carry a downstream patch there: https://github.com/gentoo/gentoo/pull/34980

It would be nice to see these fixes merged in the next release so distro patches can be dropped again.

[olifre]

@djw8605 Many thanks, much appreciated :+1: .