Crashes in tests (in random order)

[igt-fred]

This issue is a probably a follow up to #3449, I didn't dare post there (closed).

I think I've found the reason of the crashes, this seems to come from the side effect of calling spdlog::drop_all() in some tests (like "clone-logger") and then calling any method of spdlog::default_logger() (without checking it was valid) in some other test.

To illustrate it easily, just run the cases "clone-logger" and "argv1" in that order:

> spdlog-utests.exe -d yes --order decl clone-logger,argv1
0.002 s: clone-logger
argv1
-------------------------------------------------------------------------------
E:\dev\_libs\sources\spdlog-1.17.0\tests\test_cfg.cpp(35)
...............................................................................
E:\dev\_libs\sources\spdlog-1.17.0\tests\test_cfg.cpp(41): FAILED:
  {Unknown expression after the reported line}
due to a fatal error condition:
  SIGSEGV - Segmentation violation signal

This was on Windows with visual 2026 x64 18.3.2, spdlog 1.17.0, catch2 3.13.0, but I guess it doesn't matter (except catch for the random order).

Most "risky" calls to spdlog::default_logger() are in test_cfg.cpp, others seem covered by a set_default_logger().
On the other side, 33 occurrences of drop_all(), might be more difficult to fix...

Fortunately, the library itself is OK, and all the tests run fine with --order decl.
I hope it helps (feel free to close it immediately if no fix is planned).