Fix CI: do not hide base class methods in the #3989 test SAX parsers

clang-tidy's bugprone-derived-method-shadowing-base-method rejected the
test SAX parsers that derive from json_sax_dom_parser (or the test's
SaxEventLogger) and redefine their non-virtual event functions. The
recovering DOM parsers of unit-class_parser.cpp and unit-regression2.cpp
now hold a json_sax_dom_parser and forward to it, SaxEventLogger gets a
flag to recover from errors instead of a derived class, and the one
function unit-alt-string.cpp redefines is marked as intended.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-alt-string.cpp

@@ -384,6 +384,9 @@ TEST_CASE("alternative string type")
                 : nlohmann::detail::json_sax_dom_parser<alt_json>(j, false)
             {}
 
+            // sax_parse() calls the SAX parser's own parse_error(), so hiding
+            // the one of the base class is what recovering takes
+            // NOLINTNEXTLINE(bugprone-derived-method-shadowing-base-method)
             bool parse_error(std::size_t /*unused*/, const std::string& /*unused*/, const nlohmann::detail::exception& /*unused*/)
             {
                 ++errors;