Move the MessagePack SAX tests into unit-sax_parse.cpp

With develop's latest changes, the MinGW Debug object of unit-msgpack.cpp
has 33134 COFF sections on its own and 33411 with this PR, more than the
32767 that the MinGW linker of the "clang (20.1.8)" job can relocate. Its
sax_parse calls, all with utils::SaxCountdown, instantiate a parser and
binary reader of their own; moving those tests into unit-sax_parse.cpp
brings the object down to 32271 sections (unit-sax_parse: 20678).

The moved tests are unchanged: the "SAX aborts" sections, the user-defined
SAX consumer of #5405, the deep value read through the SAX interface (#5104),
and "MessagePack SAX parsing stops at every event".

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

.github/CONTRIBUTING.md

@@ -111,8 +111,8 @@ context which existing file needs to be extended, and only very few cases requir
 When fixing a bug, edit `unit-regression3.cpp` and add a section referencing the fixed issue.
 `unit-regression2.cpp` holds the older tests; the two files exist because a single one grew large enough for the
 MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing the larger one.
-Regression tests that call `sax_parse` go into `unit-sax_parse.cpp` instead: every call instantiates the parser and
-the binary reader that recover from errors, which grows a test file considerably.
+Tests that call `sax_parse` go into `unit-sax_parse.cpp` rather than into a large test file: every call instantiates
+the parser and the binary reader that recover from errors, which grows a test file considerably.
 
 #### Exceptions