tests: fix unit tests to not be affected by custom level names

The unit tests were failing when users defined custom SPDLOG_LEVEL_NAMES
or SPDLOG_SHORT_LEVEL_NAMES in tweakme.h. This happened because the tests
expected the default level names but were getting the customized ones instead.

For example, with custom short names defined, the test would fail like this:
  REQUIRE( spdlog::level::to_string_view(spdlog::level::trace) == "trace" )
  with expansion: "TRC" == "trace"

This fix undefines these macros in tests/includes.h (right after setting
SPDLOG_ACTIVE_LEVEL) so that unit tests always use spdlog's default level
names, regardless of any customizations in tweakme.h.

Fixes #3466

tests/includes.h

@@ -23,6 +23,9 @@
 
 #define SPDLOG_ACTIVE_LEVEL SPDLOG_LEVEL_DEBUG
 
+#undef SPDLOG_LEVEL_NAMES
+#undef SPDLOG_SHORT_LEVEL_NAMES
+
 #include "spdlog/spdlog.h"
 #include "spdlog/async.h"
 #include "spdlog/details/fmt_helper.h"