doctest: Do not use unnamed class

[kraj]

When compiling with clang this results in emitting absolute path into debug info, especially .debug_str section has

~(unnamed class at /mnt/b/yoe/master/build/tmp/work/core2-64-yoe-linux-musl/libcereal/1.3.2+gitAUTOINC+ebef1e9298-r0/git/unittests/doctest.h:6428:5) (unnamed class at /mnt/b/yoe/master/build/tmp/work/core2-64-yoe-linux-musl/libcereal/1.3.2+gitAUTOINC+ebef1e9298-r0/git/unittests/doctest.h:6428:5)

Therefore use a name for the class instead and help get rid of this absolute path in debug info. This fixes

File /usr/lib/libcereal/ptest/tests/.debug/test_unordered_map in package libcereal-dbg contains reference to TMPDIR

Upstream-Status: Pending

[AzothAmmo]

Looking to merge this and a bunch of other things soon but CI might be broken (unrelated to this PR) and I'll have to address that first.