doctest: Do not use unnamed class

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

Signed-off-by: Khem Raj <raj.khem@gmail.com>

unittests/doctest.h

@@ -6424,7 +6424,7 @@ void Context::setCout(std::ostream* out) { p->cout = out; }
 static class DiscardOStream : public std::ostream
 {
 private:
-    class : public std::streambuf
+    class discardBufStream: public std::streambuf
     {
     private:
         // allowing some buffering decreases the amount of calls to overflow